Honestly, as a parent it must be so scary but as teens it IS fairly normal to experiment with these things. When I was this age everyone was experimenting. I certainly did and my parents had no idea. My friends parents knew and were more relaxed about it, seeing it as just something young people try, and so my friends were much more open and honest with their parents which I always think means they were safer than I was, and also closer to their parents.
All my friends who did this are now successful, respectable professionals, so it's really not a slippery slope for most people, and it really IS common, and parents who say it isn't probably just don't know about it.
If your son goes to uni he will be exposed to all this on a massive scale. My uni was full of drugs, and people were a experimenting like crazy, and this also seems fairly common as all my friends at different unis has the same experience.
It's MUCH better to explain to your son the dangers of certain drugs, so he's aware and more sensible and cautious. If you are too hard and strict about it, he is more likely to just rebel/lie to you/feel alienated from you and therefore more likely to feel angry, upset and reckless (which can result in risky behaviour.)
Sit down and have a talk with him. Be really honest and open. Did you never experiment yourself? Tell him about your experiences, concerns, etc. Try not to explode, as tempting as I know it must be, because it's so easy to alienate a kid of this age, and I do truly believe those friends of mine who could talk to their parents about these things were so much safer than I was, feeling that I needed to hide everything from mine.