Not sure if this will be controversial... but my parents took a passive educational role and for me it worked... I stuck to weed for the most part at that age and only 'experimented' with anything else. These days I don't do anything:

Was told I could do what I liked but the house was off limits.

That I should trust the source of the drugs (did the one dealing use his own supply, did I know others who did... were there any adverse effects).

If I was going to use, do my best to differentiate between recreational and habitual (i.e. 'try not to do it on a school night')

That if I got into trouble with the law for example... then to know that my parents loved me but in this I would have to deal with the consequences myself.

Now all of this 'may' be preventative advice rather than at the stage you're at now... but a proper chat by which you speak to him as a peer and simply state that 'yes' he is responsible for his own life... and the consequences that go with it, could help.