You could also try a Limbsaver recoil pad to cut down recoil. They come in a slip-on version and there is a 1/2" thick model that doesn't increase the LOP by much. I think a pad like the LS would reduce .243 standard load recoil to about that of a .223 with a hard pad. Kids are often sensitive to muzzle blast as much as recoil so I'd be sure he wears hearing protection and, personally, I wouldn't modify a barrel or use any load that will increase muzzle blast. If he can handle a bit more weight, you could consider adding 1/4-1/2 lb of weight to the stock to reduce recoil as well.