Update: Thank you for all your replies. I can see it generated discussion. FWIW both alcohol AND drugs are threats, but teenagers will be teenagers and they will experiment - sex, too. A sense of proportion is needed. I tend to panic because I didn't have these experiences as a teenager. I was always the sensible one in the group - but my siblings did, and we lost friends to drugs, so I've always been on high alert.
Luckily, he is in a great group of friends and I can discuss ideas about damage limitation with parents. Whilst you are right, telling the school might work against his reputation, it wouldn't get him expelled because this happened out of school. It can't do any harm to raise the need for a reminder about drugs and alcohol, generally, and especially before holidays. Term time seems ok, the school is on it. I may not even need to say anything, tbh. Definitely no intention of any police involvement.
He was mortified. He said it was his first time, someone from a local school gave him some of his own stash. He told me about the experience of taking it, it didn't do much for him and he won't do it again. I relayed my concerns about not knowing if he was taking it/dealing it/part of any chain, and he reassured me that it's none of those things. I'm inclined to believe him, based on the rest of the chat and on previous discussions. He felt upset about abusing my trust. That sounds good.
We had a helpful conversation about drugs vs alcohol, which is the more damaging, whether teens have the right to experiment, the role of privacy in our family, and trust vs FOMO in general. I was able to convey that I need to keep him safe, and it's my job as his parent to make sure he's safe - he can have a good time, and we want him to "experience the world", but just to be aware around issues of personal safety. (Can you tell that this is the core of my own anxiety issues?).
Going back to my usual username now, but thank you for helping me in my hour of need and I hope this thread helps other parents.