its a good idea, and if you have clubs that facilitate it, id go for it.
boxing is great, hell learn how to defend himself, but youll end up encouraging him out of the recrnt influx of "computer children" nearly all of the kids at my college wont go outside, dont want to run around... they just want to sit and stare at screens all day.

the only thing i would add about boxing (although its not so prominent with the younger kids) is thatits more serioud than kickboxing, generally. i used to do karate, and in theory im an instructor, and i remember seeing kids get through grading and advancing in belts purely because their parents were happy just to turn up and pay for the belt.
in boxing he really will be pushed, and will go no further untill he can do it. if hes the kind of child whos easily frustrated, or loses concentration/dedication then i dont reccoment it for him


but hey, it can hurt to go along and try a few sessions eh!