These are strictly my opinions. I was a drug user for the better part of 31yrs. If I never learned anything in my life, I learned about substance use disorders.
If your son stopped for any length of time then he has control. If there was any external reason for us to use, then I believe the control would be out of our hands. You can't stop him from doing what he wants to do.
If a person is satisfied with their quality of living, they should continue living as they please. It's their life. No matter how they live their life they're going to die. So they should live it to their satisfaction.
Now what I would suggest other than treatment. Find out if he enjoys what he's doing. Ask him, "if he's satisfied with his quality of living". Normally anyone that's not satisfied, would change lifestyles on their own. As demonstrated when he stopped previously.
Once that happens that's control over the situation. Encourage him. Let him know what he's already doing, that will eventually lead him to abstinence. The biggest step is when he doesn't pick it up. Every time he doesn't pick it up he doesn't use. Every time he picks it up he will use. Let him know that. Then he can make up his own mind.
"Ask and it will be given to you; seek and you will find; knock and the door will be opened to you. For everyone who asks receives; he who seeks finds; and to him who knocks, the door will be opened." - Matthew 7:7-8