Personally a .50, even a .54 caliberwill not really kick anymore then a .45 caliber in a side lock. You just load it down. Those big heavy barrels take a lot of the bite out of a rifle. When the guys would get together on weekends, we let the kids shoot .54 caliber Renegades with 60 grains of powder, and they would beg to shoot again.

But something you need to keep in mind is weight. I would get him something like a .50 cal Lyman or I have two Traditions WoodsmanHawkins in .50 caliber and they are small, light, and with a light powder charge, very little recoil and are real accurate. I would watch the auction sites and see if you can get a nice.50caliberin Traditions, Lymans,T/C or a CVA sidelock.