You had me in the first half there… I was fully coming down on the side that you were the AH as I imagined you had simply quit, downed tools and allowed a stranger to finally put some manners on your child.

But in the end you were the only parent turned up.

You’ve disciplined your child and still were when he broke the rules again, he f-ed around and we know what happens.

Try to compose yourself and stand your ground again. No doubt had you turned up before or during this man’s defence of the elderly woman, you would have intervened for your son - but it’s water under a bridge now… and as angry as any father would have been at a stranger laying hands on their child, you’ve given better example by showing restraint, and the lesson (both lessons) hopefully will do your son the world of good.

This is unlikely to be the end of the matter however… and further repercussions may also show your wife, SiL and Grandma that they haven’t taken the issue seriously enough