He sounds like a very smart seven year old and I think it's really neat he's not playing into the game. Because he's using a coping strategy that protects himself and you that he will most likely continue to use, i.e. pretending everything is ok, it might help to try reflecting back what he says with questions, "That's nice Mum's proud of you, how does that make you feel?" Anything that makes him keep talking about it. Even though you can't legally discuss the toxic behaviors of Mum, those same behaviors come up in other people and kids TV shows (excuse the screen time plug) and I've found casually bringing them up while watching helps too, 'Wow Night ninja is being a bully to Catboy, what do you think Catboy should do?". I don't know if its helped much, but it does help me get up in those emotions a bit. Keep being the positive parent you've been so far too, that's clearly working:):).