Galahad - when I first read the name I thought it was a girls’ name (even though it isn’t classically feminine), however, I don’t think other people will think that.
I like how the connection to Arthurian legend is there but still quite subtle (I didn’t know there even was a Galahad, I expect few people do) so he won’t get comments on that all the time.
[name_m]Endymion[/name_m] - I like this one, too, for the same reasons I like Galahad for you. There is a connection to something you probably care about / enjoy reading about but it isn’t obvious to the rest of the world.
Faramir - This one’s a little too connected to LOTR to me; I like this one in the middle name spot as a “geek” name for the parents but as a first I think something a little less “current pop culture” would be easier to wear.
[name_m]Horatio[/name_m] - It’s grand but it doesn’t come with expectations; I like that.
[name_m]Archimedes[/name_m] - this one feels too much for a child or an adult really. I think he’d only ever go by [name_m]Archie[/name_m] as I can’t really see any 15 year old calling his friend [name_m]Archimedes[/name_m]. I personally know somebody named [name_m]Socrates[/name_m], so in a similar vain, and even though it’s just their middle name, it’s something people always comment on (how ridiculous it is) when hearing it.
And if he doesn’t end up being good at maths he’ll always get to hear how ironic his name is.
Hephaistion - I’m not even going to try to figure this one out; it’s always going to be misspelled, mispronounced or people won’t even struggle to the end and start calling him Heph.
Gwydion - Same as Galahad & [name_m]Endymion[/name_m]. I like it more that the former but less that the latter.
[name_m]Galileo[/name_m] - same as [name_m]Archimedes[/name_m]. He’ll likely end up calling himself [name_m]Leo[/name_m] as to not get comments from everyone.
[name_m]Lancelot[/name_m] - unlike Galahad everyone knows [name_m]Lancelot[/name_m] and while it’s cute for a toddler who loves playing knight, I can see teenagers being real jerks about this one (even if he is liked), not in a bullying way but rather by always making annoying comments ("[name_m]Lancelot[/name_m], oh my night in shining armor, will you open the door for me", said jokingly by a 15 year old).
Can see this one becoming [name_m]Lance[/name_m] as soon as secondary school starts (if not earlier).
Hyperion - this feels a bit much to me personally, but I’d say it’s still rather usable. Though I’m not sure if you wanna risk “hype” or “hyper” as nicknames.
So I’d keep [name_m]Endymion[/name_m], Gwydion, [name_m]Horatio[/name_m], Galahad & Hyperion!