DiddyW, I'm sorry you're going through this. There are quite a few of us here who have experienced violence from our teens - more than will admit it publicly. It's an awful, incredibly stressful thing to go through.
You have a right to be safe and feel safe in your own home. However stressed or unhappy your DS is, he does not have any right to hurt you or frighten you. He must learn to deal with negative emotions some other way: you can support him, but don't let him use you as his punchbag.
You've just turned a corner, though you may not have realised it. :) Deciding you will not tolerate it any more is the first step to stopping it: you have drawn a line in your own mind, and now you have to tell your son, and stick to it.
Tell him (in writing or face to face) that you have decided that you will not tolerate him being violent to you ever again, and that if he ever hits or threatens you again, you will call the police.
Then, if he ever hurts or threatens you again, phone 999. Don't hesitate. Tell them "My son has hit me. I need help" or "My son is getting violent. I'm scared. I need help".
The police will come. They will take you seriously. They will not arrest him unless you ask them to, or he has done you or someone else serious physical harm. (So it's better to call them early, before he gets to a point where he might hurt you badly). Most people's experience of calling the police in these circumstances is positive. Mine was.
My son was furious with me. But my bottom line was clear: I told him "I can't control you. You need to control ^yourself.
If you can't, I need to get help".
I had to call the police three times. The third time I had my son arrested and charged with assault and criminal damage.
It was a terrible, bleak time, but I didn't know what else to do, and looking back, I still think it was the right thing. My son was never violent to me again (it's been about two years now) and although he got a caution and a criminal record, this has not stopped him getting into college or getting a part-time job with a reputable national employer. For us, it was a major turning point - one of the key things that got my son 'back on track'.
If you want to read more about how things were for me at the time, you can do an advanced search for my nickname plus the key word 'violence'. You'll see other people sharing their experiences too. You're not alone.
While you are dealing with this, take time to look after yourself. Do some lovely relaxing things - things you enjoy, whatever those are. This is not a luxury - it's a survival essential. You need to recharge your batteries, balance out some of the horrible times, and remind yourself (and your son) that you deserve good things, not all that stress and nastiness.
Good luck. I wish you strength. :)