Everybody's different when it comes to recoil. A round I think isn't too bad might make you flinch, or visa versa. He's still pretty young, but I think that with practice he'll do fine. I'd recommend two things, first I think you're probably doing the right thing by stepping down to the .243 Win. A second thing to try is providing him with some sort of additional protection at the range. Look into a PAST recoil pad. It fits similar to a shoulder holster and provides quite a lot of additional recoil protection. I'm not sure if they make a version that is smaller to fit your boy, but it might be worth looking into. Providing the protection at the range will allow him to build his skill and confidence without learning to fear the recoil. It will also let him get used to the feel of recoil, and over time he'll get used to it and be able to handle more without discomfort. Also, when he takes his first shot at deer, as I'm sure you know, he'll never feel the recoil when he has a buck in his sights.

Good luck.

Mike