Hurryup - firstly, you must stop walking on egg shells again. Your son has seen you doing this for a long time and knows that his father was in a position of power over you. He may not know this consciously, but at some level he has internalised this dynamic. You have to remember he is a child and you are the adult.
Clearly your son is angry that you have split up and his behaviour is of course connected to this. He is too young to process his anger, so is taking it out on you, as he obviously sees you as the weak person, who is unable to control him - again not consciously - this is scary for him as well as you. He has to be helped to talk about his feelings re the split up and you need to listen to him, only asking things like "how does that make you feel" - and the answer might be revealing.
There are many children living with step fathers and it isn't always a happy experience for children. Maybe your son knows of other kids who are having a rough time with the step dad and he is scared that is what will happen to him. However you must not let your son dictate to you - this gives him more power than he knows how to handle - quite a dangerous situation.
Can you say what your son's behaviour was like when you were living in an abusive situation. The problem is that children who live in these situations are often ver damaged by it all and this comes out in anger in boys and sometimes depression in girls, though that is rather a sweeping statement.
I suspect you have been referred to CAMHS and to be quite honest I have quite a low opinion of them (they are the old child guidance people) though I know it depends on the specific counsellor. Can you or your ex afford to pay privately to see a clinical psychologist which will probably be more helpful. Really you need family therapy as this involves all of you and your ex - any chance of paying for that.
But No. 1 and I know it won't be easy you have to change your behaviour towards your son. You can't change his behaviour but you can change your response. It is a tough call because your son has seen you being a victim for so long...........but try as hard as you can to gather some emotional strength and stand up to your son. At the same time remember he is angry and confused and approaching adolesence and you need to make time to listen and talk to him.
Hope things get resolved for you all.