So, you've pushed it pretty far before and it didn't work. At this point, with someone in such a severe state of inability, there really isn't much you can do except to try to carve out a life for yourself.
First, you start to do that emotionally. Read some books on Codependency and things for families of alcoholics. I'll put some links below. As you start to gather information, you'll feel an inner strength which will begin to open up doors of power inside of you that you weren't able to access before. With that new found sense of self, you'll see your situation with new eyes and then maybe some more solutions will show themselves to you.
There isn't going to be a magic key to this problem, I'm sorry to say. It sounds like your son is seriously impaired by some problem: if you can't force him to get help, and don't want to go to the law (which would be temporary anyway), then you'll have to look at accepting what you cannot change. That doesn't mean you accept defeat or unacceptable behavior, but it does mean to accept the limits of the situation you're in.
Search online for others in your situation and get hooked up with support. It will save your sanity to talk to other women in your situation- and there are plenty of mothers with adult children who are impaired with various disorders.
I would draw the line with becoming a victim of his threats and abuse at home, though....even if that means you get to the point of involving the law. Sometimes people in his situation go further than what they can imagine and don't really want to harm people, but they do. If you think it's becoming truly dangerous, call the ambulance or the law.
Good luck in your journey. Carve out a place in life for yourself somehow - you're worth it.