You found your point of breaking and grabbed the lifeboat. Self preservation as ZOSO mentions seems to go against everything that we are as parents. But you realize that you cannot 'save' them from themselves and there is a reason that you are here and people who need you.
You have been thru hell. I can't help much but i can let you know that we all cry for each other and for the children. It's a greater pain which sees no relief. Walking away and letting him control his own life isn't easy. I have a nephew who is a handsome man. He was 18 when he entered prison. He had an attitude but managed to remain in one piece without any physical altercations or assualts from any other inmates. He had never broken the law before and had not experience with the 'life' in there. He smoked some weed but that's not what sent him there. He learned fast and adapted to the expectations. Became american muslim (like so many have to) to survive. Once out, went back to the lifestyle he was confortable with. It saved him.
This won't change your worry for your son, but i think the distance both physically and with availability has helped you greatly and he is responsible for his destiny now.
hugs to you, please keep us updated. There are many moms here who have many posts. Please read them as they have been where you are.
I will keep you in my thoughts and prayers.