At that height I'd say get him a small adult bike and let him grow into it - which will happen faster than you think!
I'd have thought that kids are more likely to be into mountain bikes or fixies than a road bike. Unless he's specifically into road racing, road bikes aren't really in fashion. A mountain bike will probably be the most versatile as he'll be able to take it over the park etc and ride it on the grass, bump it up kerbs, go on trails etc, the stuff that kids like doing. The other advantage is the low frame compared to a road bike, so you can use a short (or cut down) seatpost to start with, then put in a longer one as he grows. Another option, of course, is a cross bike which will take a bit of off-road abuse.
