I just found out that my 4th baby due in [name]August[/name] is a boy. His siblings are [name]Spencer[/name], [name]Owen[/name], and [name]Margaret[/name] nn [name]Maggie[/name]. My favorite is [name]Henry[/name], but my husband is not sold on it. Others that we are considering are: [name]Nathaniel[/name], [name]Lucas[/name], [name]Liam[/name], [name]Eli[/name], and [name]Andrew[/name], but open to suggestions.
I really like [name]Henry[/name] in your sibset! It brings out the classic charm of [name]Margaret[/name]. [name]Nathaniel[/name] would probably be my second choice because it also fits well style-wise and it breaks up the two-syllable pattern. [name]Lucas[/name], [name]Liam[/name], [name]Eli[/name], and [name]Andrew[/name] (okay, maybe [name]Andrew[/name] to a lesser extent, but definitely the first three) seem to me like pretty modern names and bring out the more modern feel of [name]Spencer[/name] and [name]Owen[/name], which sort of leaves [name]Margaret[/name] out on a limb by herself.
Other names you might like with [name]Spencer[/name], [name]Owen[/name], and [name]Margaret[/name] nn [name]Maggie[/name]:
[name]Benjamin[/name]
[name]James[/name]
[name]Thomas[/name] (love in your sibset!)
[name]Oliver[/name]
So my vote goes to [name]Henry[/name], [name]Nathaniel[/name], or [name]Thomas[/name]. Good luck! 
I really like [name]Henry[/name] and I think it’s perfect with [name]Spencer[/name], [name]Owen[/name], and [name]Margaret[/name]! I like all the other names too, especially [name]Lucas[/name], [name]Eli[/name], and [name]Nathaniel[/name], but I think [name]Henry[/name] is best. What about:
[name]William[/name] (nn [name]Liam[/name])
[name]Oliver[/name]
[name]Lachlan[/name]
[name]Edward[/name]
[name]James[/name]
Apparently [name]Henry[/name] and [name]Jack[/name] are extremely popular names in wealthy inner city suburbs of [name]America[/name]. Does this bother you?
Too bad [name]Nathaniel[/name], [name]Eli[/name] and [name]Andrew[/name] are my favorites from your list. I really like [name]Andrew[/name] for the nickname [name]Drew[/name], it sounds great with siblings names.
other ideas-
[name]Graham[/name]
[name]Emmett[/name]
[name]Blake[/name]
[name]Gavin[/name]
Your children have great names! I like [name]Henry[/name] as well as your other ideas but here are some more similar ideas:
[name]Sydney[/name]
[name]Peter[/name]
[name]Calvin[/name]
[name]George[/name]
[name]Louis[/name]
[name]Hugh[/name]
[name]Julian[/name]
[name]Thomas[/name]
[name]Simon[/name]
[name]Xavier[/name]
[name]Matthias[/name]
[name]Tobias[/name]