Well I used that rig for the kid in my avatar at 14, but he was never small in stature,, AND by that time I had him broke into 50cal ML recoil (That might be a better option for ya, can you use ML's?).
The key isn't really holding the barrel weight, Shoot off a bench when training, it's the recoil issue that can wreck'm.
Fill that synth stock with lead and spray foam, make sure it has a good recoil pad (I installed a Limb Saver) and be sure he holds it tight too his shoulder.
My experiance with the Ultra Slug Hunter barrel was that it liked lower velocity shells, stuff in the 1400 range grouped better than stuff faster than that.
He got his deer at 80 yards, just like the t-shirt say's