Good evening. Not sure if this is a good idea, but my 9 year old (4'11-5ft) son wants a single speed bike. He LOVES to ride and currently is on a Staats BMX racing bike. He rides it everywhere but he wants a bike that would be better for the street. I am not sure what to look at but I thought it would be nice if I could find a track frame that would allow 650cc wheels. He and I both like the idea of a single speed and I don't really want him on a road bike. By the way, I ride a single speed but not fixed. Any thought?