OP, I'm a little concerned that you yourself were initially unsure whether you were BU or not about this. And that may be because you've experienced violence from your brother before yourself and (I assume) your mum minimised it with you too.
Can I just say you need to get clear on what is and isn't ok behaviour within the law and child safety etc etc.
I also think you need to ACT on it. Tell your mother and brother what the law is, and make sure that they both know you WILL report him if it ever happens again regardless of your DSs behaviour, and regardless of what the family thinks is ok.
It is critical your DS knows you are solidly behind him, so make sure he also knows that what happened to him was wrong without equivocation.
I speak from bitter experience.
My uncle assumed he could discipline me with 'smacks' growing up. In the 70s, this was how my intents occasionally disciplined me too. But I loathed my uncle for it. It festered until at the age of 20 we got into a massive fight and I told him to 'fuck off'. He assaulted me badly, and it affected my life. My parents went NC then, but it was too late for me. Even then I couldn't press charges. I've never seen him since in my life though.
I don't believe he would have done that if he knew categorically from day 1 that he had no right to 'discipline' me, and if he knew my parents would come down on him like a tonne of bricks.
It's not enough to just go NC. You need to draw a line and issue a warning.
As far as I am concerned, your DSs behaviour is an entirely separate issue and not related to this at all.
I feel for you both. I'd have reported him instantly, but only because I have hindsight. I hope my experience gives you the benefit of it.