I would put the gun on a sand bag like a rifle, and have him get behind the gun right so that his weight is pushed forward, make him get off and on the gun several time's. if he is a tall skinny 9 year old, he can handle the recoil.
I can tell you your not the only one that has felt that way. I was sitting in a box blind with my runt 70 lbs. skinny 9 year old, when a nice 6 pt can out, the only gun in the stand was a hopped up 257 stw. he begged me to shoot the buck, i gave him the gun and figured what's another trip to the ER. he smoked that deer, and did not even notice the recoil. It was another memory made.