So, speaking as someone who was a bit of an obnoxious teenager who got obsessed with LessWrong way back when, myself, and who is now currently alive and non-psychotic and who has been a part of a subcommunity of LW's for over a decade, let me try to say a few of things that I think might be useful.
I would strongly recommend you read some of the sources he's citing. gwern, in particular, whom you've mentioned, has in fact done incredible work about nootropics, and more to the point his stuff is not as black-and-white as your kid sounds to be making it (although you will forgive me if I am at least a little bit sceptical of your complete characterisation, just as a matter of praxis). He does talk about the risks and side effect profiles of many of these things, it's not in fact the case that there are no negative side effects to LSD.
As a practical matter, when I was his age and going through this, nothing would work best as an argument than actually referring to the very sources I was talking about. It would have felt like I was being heard and taken seriously and engaged with on the level I wanted to be engaged with. And yes, I understand it can be a little bit of a headache to have to twist yourself into a pretzel to phrase things exactly right but in my experience it's sometimes necessary.
In full generality, I think one of the most useful things for him to hear you properly will be feeling like he is being heard and engaged with. It is in fact the case, for instance, that there are many illegal drugs that are less harmful overall than some legal ones, and a blanket "drugs are bad" stance is more likely to alienate him than to get him to listen to you. Heroin is that bad; LSD is actually not. If you go into this armed with actual data and a good understanding of risks it will feel a lot more like a good-faith discussion than a lecture and make him more likely to trust you.
Talk about harm mitigation strategies. Acknowledge that his body is his to do with as he pleases, but that as someone who loves him and wants what's best for him, it would make you happier to know that he is being smart about it. Heroin is bad; heroin with a dirty rusty needle being passed around between a group of five people is worse.
He is dead set on heroin? Fine, talk about the importance of clean non-reusable needles and access to emergency care. He really wants psychedelics? Fine, tell him that it is important to have a minder when going on trips so they can take care of him if he has a bad one or just in general prevent him from harming himself.
It is much easier for certain types of people to reject ideas if they're allowed to see them with this kind of stark clarity. I would not be surprised if, after a conversation about things like needles and side effects and addiction risks and long-term damage and all of that, your kid just decided heroin was not for him, in a way that would be much harder if he were just thinking about it as "a thing to try: y/n?"
It might be an important thing to highlight the more psychological kinds of side effects or long-term consequences and interactions. There are some psychedelics that are very anti-recommended for depressed people in particular, for instance, because they can trigger periods of acute suicidality. Permanent psychosis can happen as a result of some drugs in a way that is not well-understood and that can affect long-term quality of life very strongly.
On that note, looking into how well-researched things are is also a useful angle. Trying newfangled poorly-studied drugs out is a much more dangerous proposition than doing something like LSD which has so many anecdotal reports and analyses lying around.
Talk about percentages and probabilities with him. Yes, it is in fact the case that, qualitatively speaking, lots of legal and illegal drugs have similar side effects or risks; what makes or breaks things often is in fact the probability that something will go wrong is just insane. And even when it's not insane insane, factors of ten in risk profiles are, actually, important.
A thing that was very important to me personally when I was that age, when it came to drugs, was knowing that my brain was going to be undergoing some very extreme biochemical changes until around age 25, and that messing with brain biochemistry during formational periods was not a great idea for the long-term functioning of my brain. I am perhaps projecting a bit, here, but keeping my brain intact and unharmed and able was a very important thing to me, so arguments from that direction were useful.
Don't do anything that might look like threatening to remove privileges, like access to LW or anything like that, because that'll just immediately place you and him on opposite sides of a hostile situation. Kids will find a way to work around any restrictions you place on them, and if you want to preserve your long-term relationship with him you need to approach this as a collaborative effort towards his health, happiness, and well-being, and not as a situation where "parent knows best".
So, speaking as someone who was a bit of an obnoxious teenager who got obsessed with LessWrong way back when, myself, and who is now currently alive and non-psychotic and who has been a part of a subcommunity of LW's for over a decade, let me try to say a few of things that I think might be useful.
As a practical matter, when I was his age and going through this, nothing would work best as an argument than actually referring to the very sources I was talking about. It would have felt like I was being heard and taken seriously and engaged with on the level I wanted to be engaged with. And yes, I understand it can be a little bit of a headache to have to twist yourself into a pretzel to phrase things exactly right but in my experience it's sometimes necessary.
He is dead set on heroin? Fine, talk about the importance of clean non-reusable needles and access to emergency care. He really wants psychedelics? Fine, tell him that it is important to have a minder when going on trips so they can take care of him if he has a bad one or just in general prevent him from harming himself.
It is much easier for certain types of people to reject ideas if they're allowed to see them with this kind of stark clarity. I would not be surprised if, after a conversation about things like needles and side effects and addiction risks and long-term damage and all of that, your kid just decided heroin was not for him, in a way that would be much harder if he were just thinking about it as "a thing to try: y/n?"