One of the biggest things is putting the butt of the gun where it belongs.  If he's right handed, have him raise his right elbow to perpendicular or higher to his body and then put the butt of the gun in the pocket formed in close to his neck.  If he's holding it right he should be able to just tilt his head almost straight forward to the stock to sight right down the barrel and not lean it to the side any.  If you get the gun put there in the "pocket" you won't notice hardly any kick and it won't hurt his face either--but he needs a trap stock on it too, which is a straighter and longer stock.  Left handed, just do the opposite side the same way.  Too many people put the butt of the gun too far out on the shoulder.