I have a cousin named [name_u]James[/name_u] that has never been called [name_m]Jim[/name_m] in his life. I also have two uncle’s named [name_u]James[/name_u] that exclusively go by [name_m]Jim[/name_m]. Throughout my school career I knew several Jameses and most just went by [name_u]James[/name_u], but there was one [name_m]Jimmy[/name_m] (who I believe changed to [name_u]James[/name_u] sometime in middle school).
I don’t think that the nn will be an issue nowadays as many people have said. Especially if he only ever is introduced as [name_u]James[/name_u].