I can see someone who really wants to care for others, and tried so hard to no avail.
Understand that while your son has a drug problem, he's his own person now and needs to take his own responsibility. I know what it means to care for your son, but you can at least be proud he's making his own choices independently, even if they could be wrong for them.
Yes, I know you care for your son and wouldn't want to let him go like that, but it's his choice and as you can see there's a limit to how much you can help. You have to help yourself first.