Since you're looking for a deer gun I have to recomend that you stay away from a shotgun. Even a 20 gauge will kick hard out of a single shot. The .243 is an excellent choice for him because it's not just a child caliber. Another idea that hasn't been mentioned yet is a 30-30. A Marlin 336 in 30-30 wouldn't kick any more than a light weight single shot in .243 and he'd have the advantage of quicker follow up shots. With the new Hornady leverevolution ammo the 30-30 is now a 250 yard rifle.
Whichever gun you choose, you're going to be able to share some priceless memories with your son, congrats.