Actually, it takes only a tiny charge to get the ball clear of the muzzle. As little as 10 grains, or even less, has been used to blow out a "dry balled" ball. Load your rifle with something like 25 or 30 grains of powder behind a patched round ball to start him out. The one thing you absolutely do not want to do is to scare him with a big recoil. You can always increase the charge as he becomes accustomed to the recoil. Shooting a patched round ball is important simply because conicals will weigh more than a round ball and will give a greater recoil than a round ball. Felt recoil is largely a function of the total mass being fired out of the rifle (ball, powder, etc) and the velocity at which it is fired. Of course the weight of the rifle figures into the equation but let's not cloud the issue with too many technicalities.
I am sure he will be shooting from a bench rest at targets about 25 yards away, so a charge like 25 or 30 grains behind a patched round ball will be a great starting load and will have the recoil not much greater than a .22....well, maybe a .223. :thumbsup: