What kind of stock does the .243 have? Couldn't you just add a limbsaver to it, and increase the weight somehow? Add a bipod, a larger scope, or both! A muzzle break or suppressor would also be a good thing I guess.
If his recoil sensitivity is not caused by his physique, and I doubt that it is, then try to teach him some technique and get used to the recoil with lighter loads of .243.
If his recoil sensitivity is not caused by his physique, and I doubt that it is, then try to teach him some technique and get used to the recoil with lighter loads of .243.