(((hugs))) chic.
I don't claim to know how I would handle a child with addiction issues. My sponsor's son was a heroin addict who has now been clean for I think 3 years. To me, her situation was always more frightening than my own -- because a spouse, if you are able to take a step back, is not someone you need or have any real responsibility for. You can replace one spouse with another, to be crass. Not so with a child.
It sounds like mental illness may be more of a problem than addiction for your son, huh? "Situational depression" sounds like a copout to me -- yeah, he was homeless but that wasn't the cause of his depression; the causality was the other way round.
I know how difficult it is to find good mental health care, even if you are actively looking. Are you taking care of yourself, chic?