I love unique names that aren’t too unique or trying too hard… and I generally don’t like names that have nicknames. Middle name will be [name_m]William[/name_m].
What do you think of:
[name_m]Atlas[/name_m] [name_m]William[/name_m]
[name_m]Callan[/name_m] [name_m]William[/name_m]
[name_u]Hudson[/name_u] [name_m]William[/name_m]
[name_u]Emerson[/name_u] [name_m]William[/name_m]
Rowan William
I really like [name_u]Hudson[/name_u] [name_m]William[/name_m]. It doesn’t lend itself to any nicknames, and it’s unique but not unheard of!
I like [name_u]Hudson[/name_u] [name_m]William[/name_m]! As the pp said, it doesn’t lend itself to any nicknames and it sounds really great with [name_m]William[/name_m]!
I also really like [name_u]Emerson[/name_u], though he might occasionally get called [name_f]Em[/name_f]?
My son is [name_u]Emerson[/name_u] and was almost [name_u]Emerson[/name_u] [name_m]William[/name_m]! The only person who calls him [name_f]Em[/name_f] is his grandmother and that doesn’t happen too often 
My favorite is [name_m]Atlas[/name_m]! It’s unique, the mythological association is cool and there are going to be no nicknames!
My second favorite would be [name_m]Callan[/name_m], but he’d probably be called “[name_u]Cal[/name_u]” once in a while
[name_u]Hudson[/name_u] and [name_u]Emerson[/name_u] are not my style, but I also worry about nicknames. [name_u]Sonny[/name_u] is one that would work for both, I once met (online but still) a [name_u]Hudson[/name_u] who went by “Hu” pronounced like [name_m]Hugh[/name_m], and [name_u]Emerson[/name_u] could be [name_f]Emmy[/name_f], [name_u]Emery[/name_u], [name_f]Em[/name_f], [name_f]Emer[/name_f] etc
My choice would be [name_u]Rowan[/name_u] [name_m]William[/name_m] or [name_m]Callan[/name_m] [name_m]William[/name_m], though [name_m]Callan[/name_m] could easily become [name_u]Cal[/name_u].
[name_m]Atlas[/name_m] is my favorite! Followed by [name_u]Hudson[/name_u]. Both are nn free and very handsome alongside [name_m]William[/name_m].
[name_u]Rowan[/name_u] is one of my greatest loves, and I think [name_u]Rowan[/name_u] [name_m]William[/name_m] is such a handsome combo!
I also love [name_m]Atlas[/name_m], I think its strong and cool.
I do think all of your options are nice, though. all very handsome and sweet. [name_u]Hudson[/name_u] would probably round out my top 3 for you, mostly because it’s less likely to have a nickname ([name_m]Callan[/name_m] & [name_u]Emerson[/name_u] might get shortened!)
I like [name_u]Rowan[/name_u] [name_m]William[/name_m] the best!
I really like all your options but Callan William and Holden William are my favorites
You have some really good choices!
[name_u]Emerson[/name_u] [name_m]William[/name_m] or [name_m]Atlas[/name_m] [name_m]William[/name_m] ([name_u]Emerson[/name_u] could be [name_u]Sonny[/name_u] if you wanted) would be my favorites.