How tall is he and what is his inseam measurement?

The first thing that springs to mind is a 24" wheeled road bike but I'm not sure if he's big enough for them actualy.

Is he riding a BMX now and what size cranks are on it? If he's riding a standard BMX bike then the cranks are WAY too long and the seat is WAY too low and you could make him faster with shorter cranks and a higher seat. Slick tires would make him faster too.

You could look around for something second hand on eBay as they come up from time to time.

At his age I was nowhere near big enough for a 24" wheel bike so it depends on how big he is.

Regards, Anthony