I used to do the same thing even with a sling, then the shop owner helped me break that habbit as well as letting the bow fall naturally forward after the shot.

Short of amputating his fingers? I agree with tightening the strap a little and then have him drop out of his grip. Make him do it several times in a row. Then have him pull back with relaxed grip and tell him if he doesn't start doing it right, he'll have to get someone else to help him (you don't have to mean it, but tell him anyway). If he values what you are telling him, things will change. Some people change slower than others so it still may take some time before he becomes consistent.

Well not exactly an amputation, but maybe having him bend his fingers down against the pad just behind the fingers and tape them down. Or tape them together and then run the tape back behind the hand and wrap it around the wrist so he can't grip with them. LOL. I think it would work, but the best thing is probably blind bales before and after every practice session.

Show him the picture on pge 45 of "The Archers Bible" by Fred Bear. That's how I taught myself. If it was good enough for Fred, it's good enough for me.