None. They're his nails to shorten any way he prefers. :) One of these days he'll probably decide there's a more hygienic, more attractive, way to shorten nails.
If he asks you for any tips on how to break the habit, one is to try to only bite the thumb nails (which would leave him some nails to bite) but use a clipper for the other ten nails. If he doesn't see it as a problem, though, let him chomp them down to his heart's content. :) He's not the first, last, or only person in the world to do that.