id say let him, the verry first i used a shot gun i was 11, number 6s at a rabbit.
as long as you teach him how to stand and to take the recoil you shouldnt have a problame
the shot gu was a 12 bore as well, personaly get him to use a 12 bore, so when you hunt with him he will be able to take down animals alot easyer.
p.s first time i shot clay pigeons, i hit 4 out of 6