At his size and weight, he shouldn't have any major problems with a full-sized 12 gauge, as long as he can handle the recoil. With good padding, he should have no problems whatsoever. I'm not sure how the Kick-Eez compares to a Limbsaver, though. If the Limbsaver is better, you might consider upgrading to that.
I'm 26, 5'8", 165 pounds. I've fired my father's old Remington 1100 a few times. I'm not particularly a fan of that shotgun thanks to Remington not bothering to put a recoil pad on it at at all (wooden stock directly into the shoulder is a bit unpleasant), but it's tolerable for hunting. If it were my shotgun, I'd have a Limbsaver installed. Proper padding turns even the nastiest kick into a hard shove.