Spish,
Great advice! You beat me to it (pardon the pun). I was going to suggest renting a kit first. That way, father and son can take turns playing and if either one shows the aptitude required, then purchase a set. I played drums when I was younger and fortunately had the aptitude and feel to play. My 32 month old son is crazy about drums (Neil Peart's drum solos) and actually remembers some of the drum parts. If he is still interested in a year or two I'll probably get him a scaled down set of drums. Learning to play any musical instrument is a great skill that shouldn't be overlooked. Young musicians are the future of music.