TLDR.

There's a very similar post here: http://www.bikeforums.net/showthread...my-10-year-old

I bought a used 44cm Fuji women's bike for my 4'10" 10 yr old son (I think it's a Finest). Obviously your child is shorter so you will have a few different issues, mostly height related. My son still isn't perfectly comfortable on a road bike compared to a sort of hybrid mountain bikey thing, but he does go faster. It's still a little big on him.

So if you can hang on till he can fit on a 44cm frame you can get him into a 700c sized bike that might last him longer. Obviously there are pros and cons to waiting.