[name_m]Augustin[/name_m] is lovely!! It fits well with your other kids!!
I think great suggestions others have made have been:
[name_m]Percival[/name_m]
[name_m]Walter[/name_m]
[name_m]Benedict[/name_m]
As you mentioned he is such a peaceful calm little one how about [name_m]Humphrey[/name_m] which fits tour vintage charm, is not overused and means ‘peaceful warrior’
[name_m]Augustine[/name_m]/[name_m]Augustin[/name_m]/[name_m]Augusten[/name_m] is lovely! I have a little Swiss brother-in-law (aged 1, we have a weird family!) named [name_m]Augustin[/name_m], it’s a beautiful name.
The [name_m]French[/name_m] pronunciation is like “OH-goos-ta(n)” – I’m assuming you’d pronounce it some other way?
[name_m]Augustin[/name_m] is so cute! A great fit 
ETA: I pronounce it uh-[name_u]GUS[/name_u]-tin. [name_m]Reading[/name_m] the above, I’m not sure if I’m right on that!
It’s so great to hear from the Berries who have responded so far. Such wonderful news that you all think that [name_m]Augustin[/name_m] would work with our son’s and daughter’s name
@katinka - Good question! I live in Australia and am pretty certain that the most common pronunciation here would be [name_u]August[/name_u] (same as the month) with the -en sound at the end. The paternal side of my family were born in a [name_m]French[/name_m] speaking country so they would all pronounce [name_m]Augustin[/name_m] with the a(n) ending. My husband’s side however are very Australian and I know they would struggle with this pronunciation. It would be wonderful to be able to introduce our little guy to everyone using the [name_m]French[/name_m] pronunciation but I know that in reality, here in Australia, people would revert to the [name_u]August[/name_u]-en pronunciation and would assume that this is how it’s said if they were to see it written on paper.
[name_f]Do[/name_f] you think it’s a dealbreaker having a [name_m]French[/name_m] name that isn’t pronounced authentically as it should be?
No, I like Augustin as “August-en”. Not problematic at all.
I will say that a couple of years ago, I started a thread called “50 names to read aloud — lend us your voice!” (you’ll have to search for it, unfortunately, as links aren’t working at the moment). Augustin was one one of the names on the list, and the overwhelming majority pronounced it “ah-GUS-ten”, with stress on the middle syllable and a shortened Au- sound at the beginning.
Sadly, links to the recordings in that thread aren’t working either, but I remember this specifically because my intuitive English pronunciation is like yours, and I remember being in the very small minority with that. However, the majority of participants were in the US, so possibly not so relevant to Aus, and the “ah-GUS-ten” pronunciation is very handsome too.
ETA: I’m 100% sure you will already have considered and discounted it, but how do you feel about August as the full name? I find it a really bold, stately, solid name, traditional but cool, and with no pronunciation ambiguity whatsoever. Works well internationally, too. I absolutely love it (my fave “August” name), but unfortunately couldn’t use it myself because of our family’s little Augustin.
@katinka,
Thank you for answering my question re pronunciation! I’m super pleased to hear that you think it’s ok when the English (short Au sound-gus-ten) pronunciation of [name_m]Augustin[/name_m] is used, despite the name being [name_m]French[/name_m]. It would have been great to be able to listen to the way others pronounce this name, but it sounds like the majority of the US would pronounce it in the same way as here in Australia.
I’m glad you brought up [name_u]August[/name_u]. I find this name gentlemanly, strong and sophisticated and I also love that it doesn’t need a nickname. You’re spot on about this name not causing any pronunciation issues - thank you for highlighting this. I initially had [name_u]August[/name_u] on my shortlist before I’d even come up with [name_m]Augustin[/name_m]. Perhaps I need to give him another serious look. My hesitation with the name was that it’s now regarded as unisex. I know that sounds terrible, but it’s just personal preference. Our other two children’s names aren’t unisex and I’d just really like the same again for our third. I also wonder if [name_u]August[/name_u] sounds too [name_m]German[/name_m]? But is it wise to discount a wonderful name just for those reasons? Probably not!
I feel like I’m at the point now where I just need to ask for an honest opinion about what would be a good name for our son; one that would match well with our son‘s & daughter’s names. It’s such a huge embarrassment that he hasn’t been named yet and I wonder if it’s because I’m nitpicking absolutely every name in my hope of finding the ‘perfect’ name that probably doesn’t exist!
Some more fresh ideas would be wonderful, if there are any left?? I feel like I’ve looked at every name and still can’t find the one I’m looking for 
I just can’t find anything that I consider to be as strong, elegant, regal, noble and with a rich history as Ad3l@ide & 3dmund are (while still remaining relatively underused!?) x
[name_u]August[/name_u] is a wonderful choice and while i think it has been used on girls, I believe it’s still more strongly associated with boys. If it suits your little one, it fits your set perfectly and is charming and lovely. [name_m]Augustus[/name_m] could be a definitely masculine choice?
I’m sure you’ve heard and seen these so apologies for any repeats, but considering what you’ve said, and with 3dmund and @dela1de, I’d suggest :
[name_u]Valentine[/name_u]
[name_m]Victor[/name_m]
[name_m]Leon[/name_m]
[name_m]Walter[/name_m]
[name_m]Rufus[/name_m]
[name_m]Albert[/name_m]
[name_m]Alphonse[/name_m]
[name_m]Linus[/name_m]
[name_m]Thaddeus[/name_m]
[name_m]Phillip[/name_m]
[name_m]Barnaby[/name_m]
[name_m]Montague[/name_m]
[name_u]Lawrence[/name_u]
[name_u]Tristan[/name_u]
Gwaine
[name_m]Zacharias[/name_m]
[name_u]Curtis[/name_u]
[name_m]Magnus[/name_m]
[name_m]Reynard[/name_m]
[name_m]Dominic[/name_m]
Maybe you need to step away from thinking about names for a day or two? [name_m]Just[/name_m] so you can come back with a somewhat fresh mind.
I would also revisit the names you have vetoed but still love - maybe it wasn’t that strong of a veto!
It seems like the name might be [name_u]August[/name_u], [name_m]Augusten[/name_m], or [name_m]Atticus[/name_m]. Or something containing the es/us/est/ust sound. [name_m]Aldous[/name_m] is a great suggestion!
Others:
[name_m]Magnus[/name_m]
[name_m]Linus[/name_m]
[name_m]Ernest[/name_m]
[name_m]Ignatius[/name_m]
[name_m]Fergus[/name_m]
[name_u]Francis[/name_u]
[name_m]Julius[/name_m]
[name_m]Rufus[/name_m]
[name_u]West[/name_u]
[name_u]Ellis[/name_u]
The names I think would fit that haven’t been mentioned yet are [name_m]Ansel[/name_m] or [name_m]Anselm[/name_m], [name_m]Isidore[/name_m], or [name_m]Horatio[/name_m]. I also really think [name_m]Roland[/name_m] would be nice.
Thank you for your responses Berries 
As always, there have been some amazing suggestions!
@jonquils - [name_m]Aldous[/name_m] is such a lovely, underused gem. It has all the sounds that I love and it’s vintage and literary with a solid history. I discussed this one with DH but unfortunately it’s a little too unfamiliar to his non name nerd ears so I couldn’t sell him this one
I [name_u]LOVE[/name_u] chivalrous [name_m]Roland[/name_m], especially with the nickname [name_m]Rollo[/name_m]!! But again, this is another that my husband has vetoed x
Thank you @cactusgram - stepping away from names for a little bit was a wonderful idea!
I’m SO relieved to say that we’ve now finally narrowed down our list and are so close to picking something 
It’s out of:
•[name_m]Atticus[/name_m] - I’m still concerned it’s becoming too popular and trendy?
•[name_m]Franklin[/name_m] - not sure I love it but I do love its history and [name_u]Frankie[/name_u] is cute!
•[name_m]Pascal[/name_m] - love this and think it’s an underused gem but I worry about pronunciation issues. Would you say Pas-[name_u]CAL[/name_u] (short a) or Pas-CAHL (with the long a sound)??
Which goes best with siblings?
Any advice Berries??
Thank you! xo
[name_m]Pascal[/name_m]! I suggest this fairly frequently and no one ever picks it, but it’s beautiful, strong, dashing and with some great namesakes, too. I say [name_u]CAL[/name_u] with a short A sound. [name_u]Pascoe[/name_u] (the Cornish variant) would make for a wonderful nickname.
My second favourite is [name_m]Franklin[/name_m], but I prefer the more traditional/vintage [name_u]Francis[/name_u], especially with the sibling names. [name_m]Frank[/name_m] or [name_u]Frankie[/name_u] are very cool for short.
[name_m]Atticus[/name_m] is one I’ve never fully embraced… It’s both a bit “much” and a bit “expected” for me, but I do quite like the sound. Honestly, it does strike me as a bit trendy right now, but it’s not so popular that it will date as dramatically as [name_m]Kevin[/name_m] or [name_m]Kenneth[/name_m], and it’s a strong and solid choice with the literary pedigree that appeals to you.
[name_u]Love[/name_u] [name_m]Franklin[/name_m] for you
[name_m]Atticus[/name_m]! This has been on your list from the start, I think it works nicely with your sibset.
It’s also vintage, and I understand your hesitation for the popularity… but I’ve never come across anyone with this name… yet I’ve heard [name_f]Adelaide[/name_f] many times.
I’m equally thrilled and embarrassed that Berries are still answering all my questions in this thread.
Thank you ” At the moment it’s a three way tie between [name_m]Atticus[/name_m], [name_m]Franklin[/name_m] and [name_m]Pascal[/name_m] with one vote for each.
@katinka - Sadly, I have the same hesitations too now about [name_m]Atticus[/name_m] which is why I didn’t immediately name our little man this when my husband FINALLY agreed to using it. This reaction has really shocked me as [name_m]Atticus[/name_m] has been at the top of my list forever 🤷”*♀”
I like [name_m]Atticus[/name_m] the best. The other names sound dated and unattractive to me.
I love [name_m]Atticus[/name_m] too, but I’m not sure I’d ever use it, for the same reasons you’ve mentioned. It also strikes me as much more trendy than stately [name_m]Edmund[/name_m].
[name_f]Do[/name_f] you have a MN chosen? Could [name_m]Atticus[/name_m] be the MN?
Of your three finalists, I vote for [name_m]Pascal[/name_m]. It’s handsome and underused! It also has some good NN potential, if that matters to you – [name_f]Katinka[/name_f] mentioned [name_u]Pascoe[/name_u], but also [name_u]Pax[/name_u], [name_m]Paco[/name_m], or [name_u]Cal[/name_u].
(On that note, [name_m]Atticus[/name_m] can be NN [name_u]Gus[/name_u], which helps make it feel a little less fantastical to me. But I still vote [name_m]Pascal[/name_m].)
I love the name [name_m]Atticus[/name_m]!! I think it sounds perfect.