It's not going to matter much whether you shoot a 12 or 20 because a turkey load in either gauge is going to let him know he pulled the trigger! Start him out right away with some light trap loads and gradually work him up to the turkey load or he will probably shoot the turkey load one time and head for home! I only use 2 3/4" Winchester Extended Range #6s in my Model 1200 and they still set you back pretty good, but will kill a bird out at 40 yards with no problem with my Carlson turkey choke.

I would get him a 20ga