Help! My son is a drug addict.

I have a son, his name is Robert, he's an alcoholic/drug addict.  About three and a half years ago, my daughter called me from Los Angeles where she was living.  "Mom, I have a friend whose mom overdosed again, he needs a mother.  Would you be his mother?"  Without a thought, I said, "Of course."  I had some knowledge of Robert, he was Anna's friend, well really more of an acquaintance.  She met him while she was working at a make-up counter and he used to come in to shop for make-up and they developed a friendship of sorts.  The first time he came into the store and to her counter, he was dressed in a skirt and high heel women's shoes.  Anna is someone who can look past someone's outside appearance and love them no matter what, which she did.  So, I knew a little about him but didn't really know him; basically had met him once, but never talked. 
So . . . without further thought, I called him up and asked him to lunch.  He was ackward on the phone, shy, polite, apprehensive most likely.  I would have been, some middle age woman calls out of the blue and wants to take me to lunch; WEIRD.  But, we made plans; I really thought he would cancel.  But the day arrived, I drove to his apartment to pick him up, and my mothering relationship with him was birthed.  That first meeting was over lunch, sushi, which I didn't partake of but stuck with something of which I was more familiar, cheeseburger.  Not really.  I probably did most of the talking, let's not misrepresent here, I did most of the talking.  God's been telling me that area of my life needs work, that means basically, I need to shut up and listen more.  Neither one of us will ever forget that first fateful meeting.  We talked about, OK, I talked about guarding your heart in relationships, getting to know the other person before you give your heart away, protecting yourself, and safe sex.  I suppose those were some ackward ice breakers for a 51 year old woman and a 17 year old gay boy.  But, like I said, that fateful meeting gave birth to the relationship we now have, mother and son.  I'm not going to share at this time, all that happened up to now, I'll share that later.  But, for today, I shall share about his addiction.
He's always dabbled in substances, occasionally overindulged in alcohol.  Because of his health, he took a lot of prescription drugs which he wouldn't take as prescribed, but it was the suicide of his mother that began his quick and fateful downward spiral into the dark and seedy world of addiction.  It's been eleven months since I dropped him off in Los Angeles, California at a rehab center.  I was so relieved that he would finally get the help he needed and that I would no longer have to worry about where he was and what he was doing.  But unfortunately, that lasted only eight days.  Yep, eight days before he walked out of that rehab facility and got picked up at a Starbucks by his new sponsor, Ms. Elliott.  From that day until now, ten and  half months later, it has been nothing but a roller coaster, seeing how close he can cheat death with his wacked behavior.  It didn't take long for his addiction to pain pills to spiral into meth use and end him up in the mental hospital, twice now.  He has been to the emergency room more times they I could count on these hands, but he leaves the hospital and immediately returns, as the Bible would say, like a dog returns to its own vomit.
I have a new appreciation or rather a new hold on my heart for the parents of addicted children.  Never before have I felt such pain, loving and desiring what is best for a child, and at the same time waiting their death at any given moment and there being nothing you can do about it.  So many nights I go to bed and cry myself to sleep as if I will awake to the police at the door to inform me of his death.  And then again, time goes on and he leaves me a message; I know longer answer his phone calls because I never know which Robert I will get, the drugged out, abusive Robert, the weeping uncontrollably because of his poor choices Robert, or the I'm doing awesome, I have this many days Robert.  Each one either bringing hope, frustration, or resolve.  So, for my own emotional health, I let him talk to the answering machine.  As ridiculous as it is, it lets me know he lived through another night.