I bought him (and A.) bicycles at REI. I got him one that was on the small side for him and that had an aluminum frame and training wheels. I insisted (regular readers may recall, _really_ insisted) that the seat be adjusted quite low. R. got him to ride it and once he was using the pedals, R. and T. had a conversation and the training wheels came off. Shortly after that, the seat was raised, and they've been going for mile long rides (or longer) around the neighborhood ever since.
Very exciting! Of course, this pisses of A., because she can mostly keep up when we all go out on scooters, but not when they go out on bikes.