[name_m]Hi[/name_m] there.
[name_u]Milo[/name_u] [name_m]Oscar[/name_m] is so cute.
Here are my thoughts on your list for a little brother:
Amias-It’s interesting. It feels substantial & intriguing, but I’m honestly just not a fan of the sound. I also think that it’s a tad close to [name_u]Milo[/name_u], because it has that same ‘my’ sound.
Monty-Kind of fun & I can see the appeal. It’s sort of nick-name-y in my opinion, but I’m not sure that I like any of the full names for it better than just [name_m]Monty[/name_m].
Felix-This seems like a really natural fit w/Milo. I want to like it, but something about it seems goofy to me.
Luca-Ooh, I love [name_u]Luca[/name_u]. It’s my favorite from your list. It’s warm & a bit mysterious compared to the friendly quirky [name_u]Milo[/name_u], but it doesn’t not fit.
Gabriel-This is a beautiful name. I’m not as into [name_m]Gabe[/name_m] which may or may not be unavoidable. I love [name_u]Gabriel[/name_u] nn [name_m]Gibby[/name_m]. 
Kit-I usually prefer this as a nn, but just [name_u]Kit[/name_u] is pretty adorable as a brother to [name_u]Milo[/name_u]. [name_m]Atticus[/name_m] nn [name_u]Kit[/name_u] would be my pick though. It’s sort of a stretch, but fun, right?
Louis-I like this a lot. It’s smart & charming & kind. It’s easy & well known but not too over done. [name_u]Perfect[/name_u] fit w/Milo too.
Ellis-It’s a handsome choice. Would work well as a mn too.
A few other ideas that you may like-
[name_u]August[/name_u]
[name_u]Bennett[/name_u]
[name_m]Barnaby[/name_m]
[name_u]Charlie[/name_u]
[name_m]Caspar[/name_m]
[name_u]Dexter[/name_u]
[name_m]Desmond[/name_m]
[name_u]Elliott[/name_u]
[name_m]Edwin[/name_m]
[name_m]Fergus[/name_m]
[name_u]George[/name_u]
Gideon/Gio
[name_m]Hugh[/name_m]
[name_u]Leo[/name_u]
[name_u]Max[/name_u]
Martin/Marty
Otis…I know it’s a movie. But a great movie!

Peter/Pip
Philip/Pip/Flip
[name_m]Theodore[/name_m]
[name_m]Thatcher[/name_m]
[name_u]Wesley[/name_u]
[name_u]Walter[/name_u]
[name_f]Hope[/name_f] this helps. 