It takes a big man to admit his mistakes so that others can learn from them.
I've had 1 ND. The result of distracted loading of a single action sixgun, and a band-aid. Was loading up and having a conversation with my wife, who was in the room. I had injured my right thumb earlier in the day, and had a fabric band-aid on it. I reckon I got distracted by the conversation and forgot to skip a chamber and just loaded 6. When I went to lower the hammer, it slipped from the fabric, and since I had thought I skipped one chamber, I didn't try to get my finger off the trigger.
Result: hole through the bedroom wall into the living room and into the ceiling in the living room as I was holding the gun in an upward trajectory. Barely missed both the TV in the bedroom and the one in the living room. Thank God nobody hurt, and the only damage was the three holes, which were easily repaired.
Lessons learned were to always keep the gun pointed at something I'm willing to part with (of course I already knew this, I just got a reinforcement lesson), and even if I THINK I'm lowering the hammer on an empty chamber, get my booger hook off the bang switch immediately after the hammer starts down.
It is indeed a Christmas miracle that your son wasn't crippled or killed. Hopefully the scar will serve as an ever-present reminder to use his noggin next time. As any of us guys can attest, there must be a patron saint of doing dumb teenage stuff and not killing ourselves.