Just tell him that when he graduates to cocaine to test that shite for fentanyl and to throw it away if it has any in it.
You have roughly 15 years before he is stealing your grandmother’s silver, selling booty hole and sleeping on the park benches. Enjoy him while you can or end it now before you get too attached to him.

In all seriousness, I don’t think I know a single person that has been successful in life that didn’t do weed, cocaine and/or shrooms for a little bit in their life. The main part is that we/they all knew when to stop before they lost control. I believe that is when the “character” came out in us. We/they all said I need to stop, and we did. Some might have needed extra help but most just stopped. I can tell you this though, none of us had parents that condoned the use of drugs, and would have never wanted our parents to know what we were doing. It is a fine line in between creating a rebellion and being strict enough in teaching them what is right and wrong. You have to know when to turn a blind eye and when to say something isn't ok.

Hopefully you have been working on his character for the last 17 years. He is 17 now and he makes his own decisions rather you like it or not. It is now guidance rather than commanding. Think of it as playing pool. You might put a few balls in the pocket if you just knock the shite out of the ball, but you won’t win the game and you will look like a fool to him, unless you know how to put English on the cue ball and know when to hit soft or hard and how to help yourself out for your next shot by putting the correct English on the cue. There is also the safety shot where you don’t make a ball in the pocket, but you also don’t set the next guy up for an easy pocket.
Hope everything work out for you and the family, but I wouldn’t say too much about it. If anything just drop some hints along the way that he is slipping on somethings and ask if “you” need to help him with anything. If there is a family member that is a total frick up and drug head let him witness 1st hand what could happen if you spiral out of control. He should be mature enough to see what he doesn’t want to let happen to himself. I know I have an uncle that I won’t stop to check if I saw him wreck, but I became this way later in life after I witnessed 1st hand his shite for character, and not just my parents “protecting” me from that type of person/situation.