OP I hear your concerns. I had to re-read the thread as I started to read it a day or two ago. It sounds like your son has a pretty good head on his shoulders. He's observed a lot with his friends and is trying to figure out where he fits in when it comes to partying. I have two 18 yr olds also and we've had similar conversations here. Since last summer I wondered why I hadn't seen a couple of DS's friends much and he finally told me it was because they were out doing things that he didn't really want to do, then it all made sense to me. We wound up having a good family conversation that night, with DD too, about alcohol and how we feel about it. (Not that we hadn't had conversations throughout their younger years, but there seemed to be more context now that these issues are closer to home and they're starting college in the fall, etc.)
DH and I don't drink much. I have an occasional glass of wine, and we'll both have a pink colada or two on vacation, or a Mike's or two at a cookout, etc. (Though I have a background like you, having grown up in the city, in the 70s, and all everyone did was party. My kids have heard all my crazy stories, but recognize our world has changed now, with DUI statutes and such, and drugs that are laced with chemicals that can kill, etc.) That night we talked about alcohol having its place and the difference between drinking for enjoyment and relaxation, and drinking to get drunk. And that it's perfectly ok to enjoy a drink or two. I think the discussion had an impact and made sense, because it wasn't one of those conversations where they were both trying to run out the door, lol. They actually sat, relaxed, for quite a while and talked about it with us, so I was glad about that. We have offered them a beer or glass of wine at family events and they've tried it, but neither actually ever finished theirs. I sense DS will be like his dad, who doesn't really like to drink, and I could see DD being like me, who enjoys an occasional drink out with friends or at dinner, but my main issue, like you and all the other parents here, is just for them to be safe if they choose to drink.
That said, they've also been observing a lot. A recent college grad I'm mentoring moved out of her apartment for her senior year because her roommate was alcoholic and she was having to care for her all the time. It got to be so burdensome she moved home with her parents, over an hour away from the school. DD, though still in HS, has a good friend who's a freshman in college whose drinking has become a problem; DD's experience with this friend is starting to mirror the one with my mentoree and she's beginning to see it for what it is - lots of problems and drama. She sees she doesn't want that for herself. We've also had about four or five deaths in the past several weeks at local colleges around here - most, if not all, related to alcohol and/or drugs. Some may have been suicides, I think investigations are ongoing. Anyway, it's sad. Kids falling over ledges, drowning in streams, etc. These colleges have taken massive steps to intervene, but since college drinking is a rite of passage, it's really hard to deal with. I'm not sure a breathalyzer is the answer, but I see where you're going with it. (I remember once as a new nurse, I was discussing a patient's blood alcohol levels with a physician and I asked him what the "normal" was. His response? "Zero", lol. Makes sense. I always chuckle about that when I think of blood alcohol content.) I think some pp's are right when they say you have to trust that you've raised him right and he's had exposure and experiences, and chances are good he'll make good decisions, at least most of the time. I think this is the best we can hope for for our kids as they head out on their own.
Lastly, I do try to balance my discussions with my kids between getting a message across and not scaring the bejeezus out of them. It's not always easy, especially when we worry. Looking back, I think that's always been the case - I think of our discussions about strangers when they were little, and grades as they progressed through school, dating, etc. This is just the next natural step. For DD I warn her about being at parties, even if she's not drinking, and having something put in her drink. (I had it happen to me when I was young and fortunately I saw it. I threw up, but it could've been much worse.) I also remind her about parking lot safety and all that. Ugh, it's hard.

I think your DS will be ok.

Hang in there. Good discussion, though.