First off, I will encourage you to start him with a traditional style rifle. The traditional rifle adds a whole dimension of history and accomplishment that you just don't get with in-lines. There are some youth guns out there. But, most BP rifles have a fairly short length of pull, so unless he is small stature, I don't think you'll have trouble finding a rifle he can handle.
Anyway, the beauty of BP rifles is that you determine the power/recoil by your load. You can load as light a load as you want for practice and target shooting, then go to a heavy load when you get ready for hunting. If he plans to hunt deer with it, get the 50 cal. 45's have certainly killed a lot of deer, but anything a 45 will do, a 50 will do better. My kids hunt deer with 50's. I hunt with a 54.
This pic is my 13 year old boy (if he had any more clothes on we would have had to roll him around) with the doe he killed last week. He used his 50 cal under-hammer. We built it from a kit when he was 9 and he killed his 1st deer with it that year. His load was a .495 roundball over 80 grains of FFFg Swiss powder. The range was 23 paces and she never took a step after the shot.