I don't have a clear picture of what is going on, and I'd guess that you don't either. (By the way, I am sorry that you are having to go through this.)
1. Seek some therapy for yourself and get a professional opinion on how to approach or disconnect from this. It isn't healthy at all, and noting the fact that he is abusive, why sit and wait for something else/worse to happen.
2. Perhaps seek some legal counsel. Find out what can legally be done. If your son is disrupting your life, that isn't right. (Maybe this is his cry for help...?) And as for your ex-husband being abusive towards you or otherwise, law enforcement should at least be made aware of the situation.
3. Are you currently married to the other guy, and where does he fit into the equation? He obviously needs to be made aware of the situation.
(I'd guess the divorce or leaving the marriage is where all of the grief lies. There are a lot of emotions that get conjured up with divorce, and who knows what's been force fed to this child. None the less, there are avenues for you to take.... not everything that happens and get local authorities involved a.s.a.p.)
I think there's a lot more going on in your relationship with him that isn't related to the breakup. If he was "heavily drinking" at the age of 15, that's really a red flag that he was completely unhappy.
It's hard to tell from your short post, the entirety of what's going on here that's made him feel so damaged. Has he ever had counseling? Would he accept that?
In general, is he successful in life? (High School diploma or GED, goals, a job or college?)
See a counselor and learn how to break any communication patterns with your son that encourage him to blame you. You can't do anything about your ex and his communications, but you can control your reactions, and walking around desolated plays into both of their manipulations. Want to give them the key to your emotional house, or do you want to be in charge of yourself?