Since his first few revolutions are rolling over his thumbhole, I agree that the ultimate solution is to work on altering his release. If he was simply catching the outer edge of the thumb hole, then the drilling recommendations forwarded in Rightycomplexe's post
could help.
Another "fix" that has worked well is adding right lateral (palm) pitch to his thumb hole. However, this only works if again, he is catching only the outer part of the thumb hole.
Rolling over the thumb hole (and rolling a full roller release), is due to the bowler rotating their bowling hand SLIGHTLY clockwise (for a righty, counterclockwise for a lefty) at release. This is actually pretty common with the 2-handers and any bowler who bowls thumbless, because as the hand gets ready to release the ball, this style of bowler will instinctively try to keep the ball from "falling off" the inside of their palm too early in the release. To counteract, they will often rotate their hand clockwise (righthander) ever so slightly to maintain control, resulting in a full roller
release. Also, traditional bowlers who deploy a large degree of lateral spine tilt at release will often rotate their bowling elbow (exaggerate it ) inward, causing their bowling hand to rotate clockwise at release, causing the ball to either roll over the thumb hole or resulting in a full roller release.
The best solution is to work on using more counterclockwise turn, but only with the wrist. Hard to do but with a lot of practice, it can be done. I have used this technique with 2 of my former full roller students with great success.