If he rode a pw 250 it would not compare to that kx. It has probably the highest power to weight ratio of any size bike. It has a light switch power band. But, it can be bogged around safely. Everything torqued and with in spec, its pretty child friendly. 900 seems kind of high for a 9 year old bike, but honestly they have never changed, 5cc's and graphics? Try a sob story and 750, if that does not work, have your son pitch in his 50 bucks lunch money? Warm it up and look/listen/feel how much it vibrates. Any hesitation when the throttle is snapped? Take the oil fill plug out and smell it, burnt clutch has a rather unique odor, a ran over skunk smells better. Dads abuse them bikes more than the kids. An 85 is going to be bigger and have more torque. You do hopefully have all the safety gear?