"flying off into drugs"
First of all I'm not a parent, but I was a teenager who "flew off into drugs" and so I hope I can offer some insight.
I wasn't close with my parents. I didn't feel that they were honest with me or with themselves. They didn't give me the tools to cope with the pressures of life. They were childish adults who looked to the church in Rome for all the answers to life's questions. They expected me to be perfect, their fragile egos could tolerate nothing less. It was all about them and maintaining their illusion of being good parents in a good Catholic family.
Going through the motions of JWism in light of what you now know is not being honest with yourself or with your son. I wish I had had parents that kept it real, that admitted their own shortcomings, that gave me a mature example to emulate. Instead, I got scolded, screamed at, sent to counselor after counselor, and occasionally punched or kicked by my dad, and then he left for another woman and I was left home alone to deal with my crazy mother.
I would probably guess that if your son hangs with "users" that he has probably experimented with drugs himself to some degree. But I think that a kid who has parents that "keep it real" is less likely to make his peers the very center of his social existence to the exclusion of all adult influences, and IMO that's where the danger lies. I was the typical rebellious teen who thought that adults were completely full of shit, every last one of them.
I hope I don't sound trite, teenagers are a notoriously difficult species, so don't kill yourself with guilt if things don't go perfectly with him. Just be honest with him.