i tink it would be more effective if he did a sport, there are always kids who get muscly just from the sport they do, get him to do basketball, rugby, american football. or waterpolo thats very good.(swim incl)
ur better off getting muscle like that than with weights at such a tiny age.
my friend(he is 15) got very big muscles just from waterpolo. so i would tink sports is better right now.