I’m old enough to remember [name_u]Jem[/name_u] and the Holograms, so [name_u]Jem[/name_u] feels more female to me, but I know that it doesn’t have that same connotation for everyone. I also don’t think I’d put it on the birth certificate – I’d give him something “safer”, like [name_u]James[/name_u] or [name_m]Jeremy[/name_m], so that he has a formal alternative if he needs it, and stick to [name_u]Jem[/name_u] as a nickname.