I think Your Star, Rosie, and others have hit the nail on the head. If you set it up as "the talk," it will be horribly embarrassing for both of you. But, if you just have conversations along the line, and set things up so that he can come to you, anytime, about anything, than you can answer his questions about everything, all along the way, from drugs & alcohol to sex & dating.
Really pay attention to what he says, and ask a lot of questions, and you'll have a lot of opportunities to talk about this stuff. My daughter and I had a long conversation one night after she found condoms in her dad's car (we were divorced at that point) and was rather grossed out and traumatized by the idea of her dad using them. It just kind of happens, as long as you create some openness there for them to talk about anything at all.
One thing you need to do, though, is cultivate a very calm face to use when they are telling you something that shocks the hell out of you. My daughter never really started out talking to me about what she was doing. She started out telling me what her friends were doing, in about 5th grade. And then we would talk about that.
Try to listen about twice as much as you talk. If you respond emotionally, your kid will shut down. Just put on that calm face, let them talk, and then as non-judgementally as possible, share your thoughts.
So, when my daughter found the condoms, this is how the conversation went.
She told me the story about finding them. I said, "That must have been interesting for you." That started the blurt...and she told me how uncomfortable and grossed out she felt, and she wondered what he was thinking having those in his car. After she'd talked for a while, and I made non-judgemental responses to try and let her vent as much as possible, I said, "well, here is what I think. I think there are two schools of thought about sex. In one school of thought, you want to always be prepared, and you want to be safe, so you have a condom wherever you might need one. And, then there is another school of thought that says that sex is special, and that you should plan for it, and it shouldn't just be some spontaneous thing with someone you don't know very well. I'm more in the second category, but whatever you decide to do, at least your dad is doing the right thing by protecting himself."
Think about some possible non-emotional phrases you can use when he/she starts talking that will allow/encourage them to talk more, without asking a lot of probing questions that will embarrass them. And, try not to react too much. No matter what they say, try to be calm and thoughtful. You can definitely tell them what you think, but lectures don't work very well on adolescents. Just say, "This is how I do it, and I hope you will be similar," and leave it at that.
The other thing is...teenagers need some opportunities to fail while they are still at home, before they go off into the big wide world where you can't protect them. Failures are an opportunity to learn. So, while there may be consequences for failure, I'd encourage you not to overreact (or underreact). Just view it as a chance for them to learn something that will help them as an adult. It isn't a personal attack if your kid doesn't do things exactly the way you want them to.