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.