My first bike was a yz 125. A 125 has a suprising amount of power, i rode mine until i was 21 and it was still more than big/powerful enough and im 6'5". it depends on his size, if he's a taller kid you'll want a bigger bike like that but if he's not that big a 110 or 80 might work if you think the 125 is too much. The best way to tell is to go to a dealer near by and just look at the size of the ones your wanting to get. In the long run, a bigger bike means he wont grow out of it for a while and he'll have plenty of power to do cool jumps!