Well, the answer to your question is, "YOU CAN'T HELP HIM!" The only way you can help him is to let him go on his journey alone.
Don't feel ashamed that he didn't come to you in "his time of need" because he didn't want help from you, he wanted to get high!
The only way you can help him is to let him make his own decisions, his own choices, his own mistakes. You have to love him enough to let him hate you! Sounds horrible huh? It's the only way, believe me.
If he ever gets sober/clean, he will be the first to tell you, you probably saved his life just by sticking to your boundaries and denying him any type of help that includes, shelter, money, even food. Sad huh, but it's the only way.
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Hugs, Devastated