Your child shouldn’t be alone with your mum otherwise he’s going to experience the same verbal abuse as you.
She’s not going to change after all this time, she believes in “though love” as you said, so I’m not sure it’s even worth having a conversation with her.
you can still take him around but need to make sure you, or the father (if she is mindful of him) need to be there. If they ask why you’re not just dropping him there anymore, say he said he miss you, doesn’t want to be alone or some such. You can start dropping him alone there in a few years, when he”s 7 for ex, when you can have a conversation with him and he can hold his ground and push back against any verbal abuse. But if it were to happen again in a few years. I will stop it immediately again.
now he is just getting traumatised, you know very well from your own experience.
the ball is in your court, are you going to expose him to further verbal violence?
my situation is same as yours, and I understand for ppl who haven’t experienced it, they might think verbal violence is not such a big thing, but you know yourself it isn’t.
(and I wouldn’t push for a conversation with my own mum, for ex, bc I know she’ll get emotional, keep the hurt for years, guilt trip me with all the good things she’s done over the years, tell the whole family, so I will get calls from everyone with how much I’d upset her, etc, my children might get treated differently by family in the future, etc, it’s a whole can of worms I’d rather not open. But that doesn’t mean I won’t do what’s right for my child…)