5'1" he could fit a small 26 inch most likely, but would probably outgrow it in a year or so. I know I was 5'7" at 13 and that was were I stopped. Speciallized I know makes a 24" MTB for around $200, the seat tube on it is 10.5" if I remember correctly. So might fit him for a year. I normaly don't like x-mart style bikes, but for a growing kid they can be a good option, considering how fast he will outgrow it. Just don't go for the ones with full suspension etc, go for the basics. And have a LBS put it together properly.