How old is your son? Is his musicality average, or is he showing signs of being a natural musician - a Wiz? I think that for those children who manifest an innate serious musical talent, the piano is probably a more advantageous instrument to start with. Otherwise, if he is still pretty young (under 10-11) another great instrument to start with is the ukelele. Much easier to grasp, physically and musically, and to get right into making music with, and the chord shapes will all transfer to the guitar later. Also, it happens to be pretty 'cool' right now, with lots of younger musicians making use of them in pop songs (along with older musicians in other venues!), so it should be an easy sell.