if you are thinking of racing,no kids race 4 strokes they all race 2 strokes,(they can do nippers on 50cc 4 stroke non racing) so best getting him use too a 2 stroke, ktm adventurers are a good start point,no powerband,good suspension tuff bike,but pricey,
they dont start racing 4 strokes till they hit the 9-12 y/old 85 2 stroke/150 4 stroke
also the old yamaha pee wee 50 and pee wee 80s are a good buy