In the Need of Some Good Old Fashioned Advice

So I was well aware this day would come.  And I know I am truly blessed that my child still talks to me about this sort of thing or rather just talks to me about it, no 'still' necessary.  Tonight, I was made privy (by Gage) to the fact that Gage has drank twice (with the end result, intoxication) and tried weed once (not sure he actually inhaled).  All three events occured with the same kid(s). 

I'm trying to remember that some experimentation is normal, but I'm stumped on how to make sure he knows where to draw the line.  I'm trying not to let irrational fears create unnecessary anxiety for me. Of course I experimented when I was a teenager too, but now this is my kid I'm talking about.  And well the rules of the game this time around are completely different.  Unlike me,  who was impervious to anything bad and totally smart when it came to this stuff, I'm not feeling that same confidence when it comes to him. 

I'm grateful that for the most part he doesn't soley hang out with this particular kid.  He hasn't ditched his original group of friends (yet!).   And I know 'banning' him from hanging out with "Bad Seed" isn't the right approach. He'll hang out with Bad Seed regardless of what I say.  So what's the right approach? 

I'm reaching out to my friends who are parents of teenagers, my friends/family who have long since raised teenagers and now have adult children, and my friends who don't have kids, but were the experimental teenager.  As a parent, how do I deal with this without being overbearing and in his face thus pushing my child away or being far too lenient that I just become an enabler and unknowingly walk him down that slippery slope?   Am I panicking right out of the gates?  I'm trying really, really hard to remember back to my teenage years and trying to figure out what the best approach would have been for me.  Had I been busted. 

HELP!!