I agree with Matt D. and Billy M., both of whom have a lot of experience on this issue. If your son is safe in a piston port class, he will be safe in a shifter, even if off the pace at first. Matt’s son races SKUSA’s S5 class, and even though the motors have some restriction, he would beat most unrestricted adults. My 13 year old daughter also races S5, as well as TagJr. and, honestly, the shifter class is way less prone to accidents and bent parts.
One only needs to go to a motocross track to see what a 15 year old–or a 10 year old for that matter–can do. As in karting, most adults can not catch them. Do not let anyone tell you he is too young just because he has not reached 16 or 18.
Karting is way safer than motocross. If your son is off the pace, he will simply be behind most of the action until his speed picks up. Absolutely, if you and he are comfortable with it, then go for it!