Hey everyone!
What boy names would go nice with [name_m]Jasper[/name_m] [name_u]James[/name_u]? 
[name_m]Felix[/name_m] [name_m]Benjamin[/name_m] from your signature sounds great with [name_m]Jasper[/name_m] [name_u]James[/name_u]!
Going from the names in your signature I also think [name_u]August[/name_u], [name_u]Everett[/name_u], or [name_m]Leo[/name_m] would work really well, too. Possible combos:
[name_u]August[/name_u] [name_m]Andrew[/name_m] if you want to get the same alliteration as his brother’s name.
[name_u]Everett[/name_u] [name_m]Alexander[/name_m] has a similar feel to me as well
[name_m]Leo[/name_m] [name_m]Henry[/name_m] (with [name_m]Jasper[/name_m] as a bother I like this better than [name_m]Oliver[/name_m] [name_m]Henry[/name_m] from your signature, but it seems similar)
[name_m]Felix[/name_m] or [name_u]August[/name_u] are fantastic.
Would you prefer to stick to an alliterative theme? If so, I think [name_u]August[/name_u] [name_m]Alexander[/name_m] would be an adorable sibling. If not, maybe [name_u]Everett[/name_u] [name_m]Jack[/name_m] or [name_u]Everett[/name_u] [name_m]Andrew[/name_m]?