I like the suggestion of the .308, he can really grow with that gun (in a model 7 perhaps). Another one I would look at long and hard would be a model 7 in 7mm-08, it is a really light recoil rifle, with a good deal of power and pentration. If he can shoot 20ga slugs neither should be a problem, although I bet his Mossberg has a thicker recoil pad than a rifle does.

Gordon