Well done OP!
There are some things you need to do ASAP, and some things that can wait.
Get the foot he stamped on seen by a doctor and logged as domestic violence with your local DVU. That immediately gets you legal aid and his local Force will pay him a visit.
Make sure the male members of your family don't do anything foolish. Seriously. Get them to show your DS that real men aren't bastards. He's going to need proper male role models, men who can show kindness without seeming weak, and authority without rage. I bet your Ex taught him that the finer feelings were wimpy and girly, didn't he?
For later: you mentioned he'd done something and you knew what it was. I'm guessing mortgage or credit fraud, using you as co-signatory and misreporting your joint income. If it is, the financial institution has copies. Report it to their fraud unit, tell them you were under duress or undue influence. This has two benefits: it gives him something to occupy his spare time, and it stops him reporting you. Never ever underestimate the potential for an angry twat to take vengeance against his own interests.
The rest of it: solicitor, prohibited steps, criminal harassment if then phone calls and texts go over a reasonable limit (2. Yes really).
Courage