It is the hardest thing in the world watching our children self-destruct. I changed the locks on my doors 4 times before I told my 22 yr old AS...no more, you are not coming back. Even after throwing him out, I was still involved in his life and continued to watch him killing himself. After having several nervous breakdowns, I finally told him that he was toxic to me and although I love him with all my heart and would be there for him if he wanted to get help, he was not to call me until that time. So what does he do? He sends me text messages. "Love you and miss you. sorry for everything". I keep getting appologies, but not any mention of getting help. He does not have the desire or strength to get clean. I like to think of it as "crabs in a basket". As long as I am around him, he keeps trying to pull me down inside the basket with him. Over the course of the past 4 years, I have learned not to listen to what he says, but watch what he does. And after I saw that I was putting more effort into getting him clean than he was...I finally threw my hands up and said no more. I can't fix someone who does not want help.
Just know that you did the right thing. Try not to feel guilty about it.
Good luck and hang tough.
P.S. My son also claims that he has back pain. I also have back pain and found that yoga helps tremendously. They are looking for excuses to use. Don't "let" him smoke pot or take any other drugs. Take him to a physical therapist or pain management doctor. If he really wanted to get clean, he would go.