See if he can pick that bike up in situations he will encounter on the trail. I had thought of building a 200 for my son but the motor deal fell through, and the light weight of a 125 is better. They just don't have the muscle mass at that age to fully handle a bigger bike. My 13 yr old is 5' 8" with size 12 feet, and can ride my 250 even in technical terrain, but at 135 lbs. I doubt he could pick it up from an off camber fall. Also, I think they should all ride a 125first to learn throttle and clutch control.