that depends on your son really. the dt and the kx are two very different worlds. the kx is a motocross bike and needs to be revved high. 125s are very powerful if ridden right. if not then you will just be fouling plug all the time. you could look into the kx250 four stroke, a little less intimidating, just as fast but different power types. yamaha has a good ttr line that might suit him if you dont want him to have a motocross bike.
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