You may represent to him a part of his life that was by far steadier than the life he currently has. Someone who isn't wrapped up in craziness. Stability that he may yearn for. The problem is that you can't give him that even if you were to offer it. It is something he has to find for himself. My husband sounds a lot like this guy. Absent parents. Mom checked out most of the time due to drugs and drinking. Dad absent. He turned to the only thing he knew for comfort which was drugs and alcohol. I was relatively stable in comparison even though I am an alcoholic. I had a house, a car, a job, an intact loving family. Even then he he didn't find peace until he decided to quit drinking and that was many years after we married. It was a horrible roller coaster until then. He too would protest he had changed. He said it because he wanted it to be true, not because it was true. Alcoholics lie to themselves as much as they lie to other people.
Stay here and learn. From my own experience it would be far less painful to hold him at arms distance and not get wrapped up in the craziness. If you keep allowing him back in after he has stepped on your boundaries, he will just keep pushing the limits. Coming over drunk or high. Late night texts about how you are the only one who cares or understands. It is manipulation. Putting the burden of helping him on you instead of placing the task where it solely belongs - on him. Welcome to SR. Try reading on the family and friends of alcoholics and the one for family and friends of substance abusers too. Those forums are very supportive and informative.