You mentioned he's getting bigger, I'd go with a twelve. He can become a discriminating sportsman later, for now maybe he'd like what he thinks is a grown up gun better. I wouldn't stick him with a pump action unless he wants to use it because he'll be looking around at what others are carrying, and you can decide what shells he stuffs in it. I vote, if you're thinking about buying anyway, take him looking and steer him towards choosing 'his' O/U gun.