It would be best for you to start your own thread @Mummymonkeysocks2012 as this is one from years ago and people will respond to the OP rather than you.
But just my thoughts - your son is living in an abusive home with a man who abuses his mum.
You say you can't leave this man, but also say this:
I have a son (not his) - and my heart is breaking that he is witnessing this behaviour. What if he thinks it’s ok to treat women like this? I couldn’t live with myself.
You have to leave. You are exposing your own child to a dangerous, abusive, toxic environment and teaching him that is normal.
He's alienated you, and therefore your son, from your whole family. He's made you financially vulnerable. He's exposed your child to abuse and in staying, unfortunately you have done so too.
I don't say this to make you feel bad, I say it to try to get through to you the fact that if you genuinely want to put your son first then staying with this man isn't even an option.
Call women's aid asap. Ask for their advice on how to remove yourself from this situation safely, they can advise next steps. And when you have done that, call your mum. She will be thrilled you are going to remove yourself and her little grandson from the orbit of this monster.