Yeah theres no age / height limit but common sense. The official blurb is that they can reach the footpegs & wear an approved helmet. Shouldnt be difficult for him reaching the pegs as they're designed for midgets anyway lol! I used to ride pillion on my uncles at about that age!
I was on a bike in the womb. On the fastest bike in the world at the time (GPZ900). Not sure how old I was when I first went on the back, but was young as I can not remember being too young.