How good is he already? We had a look for my 7 year old and it seemed as if the smaller (<20in wheels) were just not really good + strong enough for the kind of thing that he wanted to do. We are waiting for a year or two until he gets big enough for 20 in wheels, Meanwhile he is having great fun and giving me heart attacks at the skate park with his scooter
My biking mad DS trashed several bikes before we got him a Mongose for his 10th B'day (but is should last him forever). DS 2 was given a cheaper BMX for his 8th birthday, but still a good bike with 20 inch wheels.
Not sure which are the best with smaller wheels. How big is he? Could he cope with 20inch wheels? Sorry, not much help. (Just think I know about bikes because I have about 25 in the garage )