RE: .50 Hawken load for teen..........
If this kid can shoot a 45/70, he can shoot a Hawken loaded with 80 gr and a maxiball, no doubt about it. It may even kick less given the weight of a hawken. More of a shove than a punch too.
Sounds like a mental thing, maybe because the gun looks "big"? If he wants to practice with a lighter load just to get used to the gun that's not a bad idea.
I'd go back to the 80 gr/maxiball load for hunting. Heck, he won't feel that shot he fires at a deer anyway. Better to ensure a pass-through and create a bloodtrail than to risk losing a deer with a young hunter.