I had the same issue with my ex. Things never changed after 8 years until one day I woke up and realized that his problems were not my problem. I accepted his problems as my own which then made it our problems, and I was the one who had to be understanding and nurturing while he kept doing what he was doing, so one day I woke up and gave his problems back again and the kids and I walked away. Sure, it gave us a whole new set of problems, but there such freedom in that decision and the best decision I ever made. I am now with the man who literally is the other half of my soul and we are all happy. people come into our lives all the times, more often then not, not for the reasons we think, but it benefits either us or the other person regardless. Life never guaranteed that pain wouldn't be included, but it's a price that gives deeper meaning to everything we do.
Your not meant to save him, he must save himself, you are however, meant to consider yourself as highly as you do others. It's essential. You stay or are bound because your afraid of what will happen to him but will happen if you don't? How many times will he have to repeat the same karmic cycle because we choose to not let go when we know it's time? It's like a child. If we keep our children small, they will never grow to be responsible adults.
You have a very nurturing soul, channel that energy into someone who not only needs it but wants it.