The extensions are the key, but in all likelihood you'll want to mod the shock to accommodate the longer length...otherwise the back end will be like a pogo stick, and it'll just eat rear fenders. So at least a heavier shock spring, and ideally an adjustment to the valving to keep pace.
Other than that...it's really up to you. You're going to be competing against wildly modded bikes, that have been ported, had the head reshaped, different/bigger carbs, reeds/cages, exhaust, etc. At the end of the day though, you still have to ride the thing...so unless it's clear that the bike is holding your son back, you might wait until it's clear that he actually needs (and can use) more power. No point in spending big money on a machine if it's just going to make it harder for the rider to master...and that's what typically happens when you go hog-wild modding a bike before the rider needs it.