John is a plain name, I admit...but it's also a strong name, and a name that you can't really label because it could be anyone. If you want to use
Jack as a nn, that's fine, but I'd wait and see if the little guy seemed more like a
Jack or a
Johnny or a J_ (whatever his middle initial was.) And you can have fun with a middle name for
John because it's kind of blank slate...(And I'm not fond of
John as a nn for Jonathan...Jonathan is kinda '70s to me.)
John CharlesJohn WallaceJohn AldenJohn PrestonJohn Amos