I really can't say much more than what's already been said. I have a nearly 27 year old addict daughter, who is back in our lives again after nearly 4 years away. And the ONLY reason she is clean right now and back in our lives (her behavior tells us so) is that SHE did it! We did not! She did it for herself and for her own reasons.
I was fortunate; I learned early on in her addiction that I could and would not give up my life to try and control hers. Her life is not for me to control; it's up to her and her HP. I am responsible for no one but myself and my actions and I am accountable for the consequences of those actions. She and your son deserve the same even when it hurts us.
You do have to come to the conclusion: You cannot control the behavior of your addict; you can only control how much that behavior controls YOU!
There used to be a sticky "A message to new parents of addicts" but it also applied to the parents who are not so new to the addiction of their addicts. It said that to try control and try to "fix" your addict will make you as crazy or crazier than they are.
In my estimation and experience and knowledge (and I've had a few sessions with a family drug counselor) an addict cannot and will not recover unless that is what the addict wants and wants to fight for. Unfortunately, some never do. But it will NOT matter what you do or don't do; it's not yours to control.
I loved my daughter when she was using but I hated the addiction and behavior and could not have a front-row seat to it. As long as drugs were in her life and she was using, we kept our boundaries in place and we could not be with her.
I hope you can understand that you deserve a life, too. You have choices and you are responsible for making them.
Hugs,
Marteen