Hello and welcome. Late to the conversation but wanted to add my two cents.
I am stepmother to two wonderful, and deeply troubled, teen sons. The oldest is 18 and the youngest is 16, soon to be 17. We are not their custodial parents. At the present time we have no contact with either child by their choice.
We have gone through everything imaginable with these boys - there is a novel's worth of posts here detailing all of it - and so I understand, along with so many others who have responded here, what you are dealing with. Although everyone's personal circumstances are unique, there are components of this nightmare which are truly universal. The embarrassment, the judgment from outsiders (including those in our own families), the self-blame, the rage, the fear, the questioning of our own actions and motives. Did we leave him alone too long that day when we were late getting home from work when he was three years old? That sort of thing.
I agree with everyone here. Your son's situation is not your fault or your responsibility. My W and I have dealt with the same feelings. Our situation is somewhat different in that my stepsons do not live with us and never have. They live with their enabling father who is a very sick individual and is content to have these boys rot under his roof for the rest of their lives, as long as they do not leave home and abandon their father.
I don't want to hijack your thread so I will not repeat my family's story here, but I will say my oldest stepson DS is on the same path as your difficult son. The difference is that DS is permitted to do what he pleases and always has been. Because we feel as you do that children must be productive citizens, DS has shut us out of his life. YS is slightly more functional than DS but is not doing particularly well, either. My W and her ex-husband, the boys' father, will never be on the same page and the children have aligned with their father against her.
I am happy for you that you have an opportunity to protect, parent and guide your difficult adult child in a way we likely never will. I will pray that your son eventually gets it and turns his life around. In the meantime please stick around.