This is from the book Atomic Habits regarding soldiers in Vietnam that got addicted to drugs..

"Robins found that when soldiers who had been heroin users returned home, only 5 percent of them became re-addicted within a year, and just 12 percent relapsed within three years.

In other words, approximately eight out of ten soldiers who used heroin in Vietnam eliminated their addiction nearly overnight. This finding contradicted the prevailing view at the time, which considered heroin addiction to be a permanent and irreversible condition. Instead, Robins revealed that addictions could spontaneously dissolve if there was a radical change in the environment. In Vietnam, soldiers spent all day surrounded by cues triggering heroin use: it was easy to access, they were engulfed by the constant stress of war, they built friendships with fellow soldiers who were also heroin users, and they were thousands of miles from home.

Once a soldier returned to the United States, though, he found himself in an environment devoid of those triggers. When the context changed, so did the habit."

As I said before I'm not at all questioning your parenting. I wonder if your son might have some sort of undiagnosed anxiety or mental disorder and he is using drugs as a unhealthy coping mechanism to deal help him deal with whatever he's struggling with.

The story above shows that people will do drugs to help them cope with stress but when the stress is removed they can quit. I'm not saying anything you're doing is causing him stress. I'm saying there might be something like an undiagnosed anxiety disorder that could be the causing you son to reach for drugs.

I know I had my own mental health struggles in teenage years. Thankfully video game addiction was my drug of choice.

Hope this is helpful. I found the Vietnam story very illuminating in helping me better understand drug addiction.

I wish you the best of luck with your son!