(((NSW)))
My niece, Brit, is 15. Her dad is an A. We never bad-mouthed him in front of her, or told her of the things he did, unless she asked, and she has only recently begun to ask if things he's said are true.
She has found out, on her own, what kind of person he is. Her mom died, in a car wreck, when Brit was only 1. She has a lot of anger, and takes it out on my dad, stepmom and me, because we are "safe"..she knows we will love her, no matter what. I finally learned how to set boundaries with her (thanks to you all), on what type of behavior I will allow from her. That was after many, many screaming matches with her, that accomplished nothing.
I can't control the way she acts with my dad and stepmom, and it is their house. Honestly, I didn't think she'd even care when I detached from her because I was sick of her behavior (cussing everyone out, throwing tantrums). But apparently, she did care, and we have a really great relationship, now.
My feeling is your GS already knows what this man is like. He's at the age, where he's confused, angry, and doesn't know how to deal with all these feelings, and he's striking out at the one person he feels the safest with....you. It's not fair, but it's what we all do. I would go back to the boundaries, because I think kids need that...a lot. As much as they rebel against it, subconsciously, it makes them feel more grounded. Brit has told me there are times she really gets mad at me, but she knows that what I do, I do out of love.
I'm so sorry you are going through all of this.
Hugs and prayers!
Amy