Im 22. Smoked pot for 9 years with many other drugs included.

I have seen what being an addict has done to my mother. Although he may not acknowledge it in your presence, what you tell him about how you feel is filed somewhere in his mind and he will have to deal with it some day. It may be 5 years from now, but nothing you said to him will be forgotten. Even in my most heavily induced drug moments, the tears in my Moms eyes still had a profound effect on me, I just smoked those feelings away and continued to argue....I was so immature.
He wants to argue and justify his position, but when he realizes emotions are nothing that can be debated and he made you feel that way , whether it was justified or not(i think not), he will feel horrible. I dont know him but thats exactly what happened to me. My mom told me every day what I was doing to her and how it made her feel. For the longest time I tried to argue about it until I realized I was out of line. Your feelings are there and very real.
I can not promise that he will come to my same conclusion, but our stories sound very very very similar. Even they way my parents treated me sounds the same.
My one suggestion is this. If he tries to come back home to get sober or save money whatever, make a contract. Put everything that he needs to do in paper and have him sign it. Even include what happens if he breaks certain parts of the contract. leave nothing out! That way if he does come home again and starts down the dark side, you can just refer him to the document he signed. Its not open for discussion, these are terms he agreed upon when he came back. He can not blame you for making him leave, he cannot try to argue with you over the severity of the transgression, there is no discussion necessary. Its all there in black and white. This worked for me. It transfered the blame from my parents disciplining me, to me breaking a contract I agreed to. It seems simple, but it worked for me.
You can send me a private message using that button at the top of the page and ask me any specific questions you want. Maybe I can give you a little view into the world of a 22 year old, 9 year smoker, living at home, with tearful parents that also gave me support.
I was in the same position as your son. I made it out. Your cause is not lost.