With about 10# you could probably make it what would seem very narrow, about 1" or maybe less. Backing, I wouldn't recomend boo on a bow that small, plus red oak isn't very compatable with boo. Linen or practicaly any other fabric will make a more suitable backing. Even old cut up jeans would work. I would use just a rasp (wood file) that you can find at any hardware store. For glue, Titebond 1,2 and 3 work, and I've heard that Gorilla glue is not very good for jobs like this, so stay away from that. The length is determind by how long a draw length you are looking for. Typicaly a little more than twice the draw. Give quite a bit of extra for him, so he can grow into it.
Sean