That's about how my shoulder looked first time I shot s15-20 rounds through a .45-70 . It was sight in, squeez the trigger, hear the bang, feel the recoil,...wait for it...wait for it...OW. Reload, repeat. It was interesting watching the colors change over time.
Looks like he missed that pocket we were taught as kids.
My daughter dated a guy who worked and drove for a moving company. He was big and strong enough to carry your refrigerator in without a dolly. Yet let him sight in a 12 ga slug gun and he wined like a baby. I had to get the scoped .22 out for him to practice shooting with a scope. I wonder if it was aggravating the forming arthritis in his shoulders from all the abuse he gave them moving heavy furniture.
I believe your son misinterpreted the meaning of “shoulder the gun”.
The only time I had bruising shooting a gun was with my Contender handgun with a 14” 45/70 barrel. It was my left temple that got bruised when I was shooting from a bench with 405 gr bullets and I didn’t have my arms stretched out far enough and upon recoil, the blasted thing hit my head. Good thing that no one was watching at me at the time. There was a reason I called it the “Ouch Meister”. Death at both ends of that hand canon.