All teenagers drink when there´s a party, I did too. You should see a midsummers eve here in sweden, the hospitals are packed with drunk kids.
Both my parents know he´s been drinking, he barfed all over his carpet a few weeks ago and they don´t like it and they´ve told him but he does it anyway.
Kids in the US smoke pot and kids in sweden drink alcohol.
Tell him that you are going to turn the car keys into the police yourself if he doesn't get his act together! He's 16! He doesn't need to be doing all that crap!