This is America so it's very strange from my perspective: 16 years old has (illegal) access to weed but cannot legally buy beer.
When I grew up, getting alcohol as teenagers was a non issue. If you had money, they'd sell it to you: on the terrace of a bar in plain sight. I remember getting beer and wine when partying at my friend's houses (and them in mine) as old as 8th grade (probably around 14 years old). Parents just didn't care about it as long as you would not show up drunk.
Cigarettes were a far more sensitive issue with some parents getting hysterical over their kid's attempts at smoking (which never helped). Overall I found out that the most effective approach was trusting the kid - and making sure you tell him that - so he feels good about it. They're going to do whatever they want anyways, weather you agree with it or not, they'll just hide it from you.
Related to @LacesOut comment on "nicotine is plainly the most addictive substance known to man", I think it's a myth used to scare kids away from smoking. I could have smoked all I wanted and so all the friends I knew in my generation - very few did. It's not like I didn't try it - but it just didn't do anything for me the way a few beers would do. None of my current friends smoke and I asked them why - same experience as mine. Some people might be more susceptible to nicotine maybe?
In conclusion, impose boundaries but don't forbid, so you still get to know if he's consuming: "You will not arrive at home drunk. You will not smoke at home. Otherwise it's your choice but I recommend moderation in drinking and complete abstinence on smoking".
Also @KCalhoun is right but there is little you can do about controlling your kid's entourage. Especially at this age coolness and impressing girls is a huge factor so it's tough - good thing is that it's almost surely just a phase and it will pass.
