When I was that age or thereabouts I saw a punch as carte blanche to kick the crap out of whoever it was.
Not long after that, as I grew up, I would avoid fights entirely - despite being one of the biggest and strongest kids in the year. I calmed down a lot as I grew up and learned about myself.
I'll say well done to your lad for not taking it and it might make the other lad think twice before 'setting about' anyone else
Massive fallacy that. No-one beat me in fights, but it made no difference. Still got into loads (as a kid). If you beat up a bully boy kid they tend to want to get their revenge on you, or at least bolster their position by picking on other people.
You can't change a violent angry person with more violence.