Not easy, he sounds lovely though.
You haven't had a knee- jerk reaction so it sounds like you want to deal with it calmly and factually. ( I'm terrible at this myself but working on it).
Whatever you think of the drug, he is already smoking - the issue is how to handle that. And to limit the effects.
You can't alter the fact that he's had a smoke, and you may or may not be able to stop him but what you can control is how you deal with it. And also how you help him to think about it.
On facebook there's a page called Humans of New York, there was a post this week about an elderly woman who told her kids, 'you can get stoned but don't be a stoner'. ( I think she smoked with them too but that's another chapter)
The reactions to that are worth reading. Apart from a few extremists in both directions it makes for helpful reading.
Is that really the point? that a lot of us dabble in things that remain a dabble but what we worry about as parents is the slippery slope and if it gets serious we will wish we sorted it out earlier. Which is why you mention the gap year - I imagine the fear is about it passing in a haze of smoke?
It's not ideal but perhaps have a read yourself first, and if you feel comfortable show him the FB post and the thousands of different reactions to it. He can weigh up the pros and cons without it feeling like a lecture.
This post also had me thinking about my youth and hiding everything from my parents. If our kids our talking to us - whatever horrors that throws up - is that an advancement because they are acknowledging that they are out of their depth and need us?
Re the peer thing, sometimes teens want to stop something (this is from my memory of being 16) but they need an excuse. The best way to break a habit is to changes routine. Can you take him away or send him off to stay with someone. If he is keen to take up a suggestion it may suggest that he wants to stop himself.
I hope it all works out. I'll stop now - I've had a shocking morning with my 15 yo. Perhaps you'll have advice on my post!