I would not change the rifle into something else, your son will never have another chance to pass on "his grandfathers" rifle to his kids. My oldest daughter killed her first game with her great-grandfathers .270win, the youngest killed hers with her grandmothers .244rem. They plan on passing these guns down to their kids because hunting is not a sport - it is a tradition.
If you want to build a rifle for your son start hitting pawn shops, pretty much all you are after for a custom gun is the action and for a couple hundred bucks you will be set.