How about just teaching him to shoot with a more heads up attitude like many of the old timers shot (and me).
That would work, for sure, but if he shoots other shotguns that don't require, or need, for him to see so much rib to shoot flat, he is in trouble trying to switch guns. Much better to make the gun shoot to where he looks, with his style, as long as that "style" is not poor form. Sounds like he has a pretty consistent mount, by the way the pattern moved left when you built in some cast.
There are several ways to build up the comb, mentioned already, but it comes down to whether or not he is bothered by how something looks. I've built them up with layers of cardboard and brown duct tape, too. But, how well a gun fits me is much more important to me than how it looks. "Pretty is, as pretty does" .......... for some of us.
Best, SRH