Stop focusing on your anger towards H and start focusing on your kids. Being angry at him won't help your son, and as a side note - it won't help you either. We are in similar situations - I have kids the same age as you except I have girls. My D11 is doing well. My D14 is struggling. Every night, when I tuck her in, I lay with her and let her talk. Mostly it is just about school and friends, etc. But sometimes she will let me "in" on what is going on in her head about H. I don't force it, but I give her the opportunity to discuss it every night. It is safe for her, we are in the dark, I can't see her, etc. It seems to work.
I never refer to H as your dad to them. I never bad mouth him and I won't have discussions/arguments or whatever in front on them, and I co-parent with him. For example, homework wasn't getting done when they were with him, so I called a meeting with all of us and H and I talked to the girls about how this was going to change. I didn't ask H about the meeting - I told him about it.
Not good DBing, I know. But when it comes to DBing - it goes out the window when it comes to the kids. Many here may disagree with me, but my first responsibility now is to my kids.
I have also asked both of my Ds if they want to talk to someone. They both said no. I watch them closely and I know D14's triggers. And I usually try to avoid them, or deal with it head on.
I don't know if this helps, but it seems to be working for us.
