Hi vonnie, I missed this thread last week. I'm sorry you are going through this with your son. It happened to me too. It's more common than you think - IME, people don't talk about it, but I found that when I (carefully) started mentioning it to friends with older teens, a few of them had also experienced similar violence. :(
It's devastating and so difficult to deal with, because you can't help feeling that it must somehow be your fault that your own child is abusing you, and you remain responsible for him. It's different from domestic violence from a partner, because you can't leave your own child. There's also no support out there - I tried to get some from women's aid and a local charity, and they just did not know what to say or do.
You have a right to feel safe and be safe. Obviously, you can't control your son at this age - he'll be bigger and stronger than you, and less inhibited. He needs to learn to control himself. But in the meantime, if he can't, you have to call the police to control him and keep you safe.
To do this, you have to draw a line in your head. You have to say to your son - and mean it - "I am not putting up with any more violence.
It stops now. If you hurt me or threaten me again, I will call the police". Then you have to do it.
You probably won't have to do it very often, but if you have already called them a couple of times then there won't be so much 'shock value', and you might have to actually have him arrested, rather than just taken to his gran's.
IME, the police are pretty good in situations like this. I think they see a lot of it. I called them 3 times, and the first two, they just gave him a talking-to, and then went away because I didn't want any further action. The third time, I asked for him to be arrested. We had a truly awful week because he ran away and hid for a couple of days, but then they found him and he was arrested and received a 'final warning' for assault and criminal damage.
He was never violent towards me again.
It took a few more months for him to deal with his anger towards me. He kept saying things like "I can't believe you got me arrested, you bitch" and I had to say (as calmly as possible) "No, you got yourself arrested because you were violent. And if you are violent again, you will be arrested again". I also put up with him breaking things for a few more months, while he learned to control his temper properly.
The good news is, teens do seem to grow out of this terrible behaviour, if they are challenged. My son hasn't been violent for a very long time now - well over a year. :)