My son was a heroin addict, living on the street after we kicked him out of the house. Sometimes he parked his car in front of our house. He would pass out in his car and we would never know if he overdosed or not. It was devastating.
In the 2019 movie, “Beautiful Boy,” the father asks his son, "Why do you do all these drugs?" The son answers, “I don’t know why, when I tried it I felt better than I ever have, so I just kept doing it.” The takeaway here is understanding, “Why,” we need to feel better, the specific circumstances that makes us feel bad and, "How," to manage our feelings and thinking in a direct healthy manner.
The best you can do is love him without enabling him.
Today my son is seven years sober. He changed when his values trumped his addiction. A loving family is valuable.