Breakfree, I am so sorry you feel trapped like this. I'm sorry can only give quick reply at moment, getting dcs off to school and nursery, then work, but will check back later. I have been through similar, house was in exh name and he thought he had right to stay there, but not council, so not sure of regulations. You could try phoning or website of Shelter (housing) www.shelter.org.uk. 0808 800 4444. or women's aid - www.womensaid.org.uk - 0808 2000 247.
When I first said I was thinking of spliting up, exh twisted arguments to make me stay, he actually said later it was only because he could't afford the house and to live without my money. (and children's money in fact!) After that point when he said he needed my money. It still took me about a year after that to get away. I so much regret not leaving sooner, especially effects on oldest dc (my child, not his), I thought of christmas too, but that christmas with him was horrible, and the following christmas when we had got away was peaceful and so lovely to be relatively free of him.
Your son has said he wishes dad would just go/stop shouting, so if you did get away before christmas he would be relieved to have christmas with you, step dad not there. Your toddler IMO is too young to look back and remember it as the christmas of the split or when somone left. I realise it is very close to christmas now to get him out or leave. Unless you could stay temporarily with parents once it is school holidays?
I'm sorry I have to go in a few minutes, might be chance to write something before work, otherwise will be later. Stay strong. You can do this, it is just a matter of working out how.