OH that's a good one.
I was suspecting he somehow had the wrong guy - like a secret twin or improbably look-alike namesake - or that the hit with the rock gave the bully amnesia.
It sucks, but school bullies (and similar abusers) will rarely apologize. They usually grow up and decide they had Excuses(tm) to be that way, or somehow justify that it was the "normal" thing to do towards their victims.
It made a lot of sense that the protag, the guy who would blame his initiative to divorce AND his cancer diagnosis on his trauma, would also be the kind of guy who would turn other people into punching bags. Being a victim and genuinely having trauma doesn't exempt one from being an asshole.