Personally I would limit him to the distance he can effectively shoot in your particular hunting conditions. So I would test him on that.

I would also be more concerned about practice than I would choice of bullet. Especially at the distances we are talking about.

Open sights is also a great way to learn to hunt. So I might also keep the open sights if he is effective enough at 50 yards to kill a deer.

Second I would buy a couple of pounds of 2F Black Powder and a couple of hundred round balls and patches. And let him shoot with a 50 or 60 grain load until his heart is content. Then hunt with that same round ball and 50 or 60 grain load. That round ball and load is more than effective on deer to 50 or 75 yards.

Why waste good money on powder and special bullets when a good old PRB will do exactly what you want to do. Tom.