I have a close relationship with my daughter she lives at home and sees her childhood completely differently.
It's not uncommon for siblings to have a different perspective.
It's unlikely they were treated exactly the same way by him; they were different ages; your relationship with each of them will have been different because they are different people and their different personalities will determine how they feel about your handling of the situation.
That doesn't mean his perspective is wrong or less valid.
It's just different. You clearly feel he is being unfair. It's not his job to protect you, though, it was your job to protect him.
I agree that his relationship with his girlfriend has probably revealed a different type of family life and that is the type of family he wants in his life. He is possibly taking time now to process his childhood experience and how he feels about it.
This low contact situation may well ease over time but you will need to take his lead on that.
It's different for me. My dad didnt cheat on my mother until I was 17 so I was still being hit, locked in cupboard and dragged upstairs etc until adulthood really. During that last conversation with her (in my late 30s), I was really trying to resolve things but she sneered, she mocked, she belittled and then she pitied herself.
If you ever get the chance to speak to him about this, I would counsel that you listen. Don't explain or justify. The small boy still inside him needs his mummy to listen to him and respond with love and empathy. He doesn't need to feel he is expected to make you feel better about it.
If you try and make him understand your perspective, then you will just push him further away.
He will need to know that you hear him.
But wait for him to come to you. You had your chances and the space you make your choices. Now he needs his.