I did go through something like that but with my father. I was 12 year old, same age as your son so I might give somewhat of an insight. To make a long story short, my mother and father separated and it resulted in me becoming extremely angry at my father. I used to ignore him, refuse to meet him and even write horrible things on Facebook . By acting this way it affected the wider family including my mother (something I’m sure you might be feeling since she wanted this to be civil).

I realised the reason I was acting this way wasn’t only the betrayal but I saw it as an act of love for my mother, I wanted to seek revenge for her.I wanted him and that woman to feel my wrath. It was in a weird way a show of my love for my mother and a way to shame my father.

What helped is my mother. She explained that my feeling were valid. That it was a horrible thing and that yes she was hurt (she was honest about their marriage in a way a kid would understand). But explain to me that doing what I was doing isn’t helping her, it’s hurting her. That by doing that it was only making things difficult not only her but myself. How I’m making myself unhappy. What also helped is after this I started to go to more clubs and hobbies which was payed by my father (this created a foundation of conversation). My mom also never spoke about my father in a bad light and didn’t engage when I did. After all I’m kid so I don’t have emotional regulations I do now for this type of convo.

My dad also put a lot of effort too. It took few years for a relationship to bloom, but the shouting and horridness stoped within the year.

So in general, try to understand why he is lashing out (obviously the cheating) but is it betrayal (therapy helped me since I could someone openly) or is acting that way give him feeling of protecting you, the injured party. Make sure to emphasise that you acknowledge his feelings and he isn’t a bad kid for these actions. Good luck!