Marie has great advice. I was also the smart kid who did dumb things. The thought of my parents being disappointed in me was the biggest motivation for not going overboard. You don't have to say how you know, but reinforce your rules, reasons, disappointment and sadness. He may not stop everything straight away, but it will definitely encourage him to minimise the smoking/drinking. A big thing for me was I was not allowed to stay at parties overnight, but my parents were always volunteering to pick me up at 12/1/2am. It must have sucked for them but it certainly stopped me going way overboard! If I wanted to stay at a friends house, they would chat to the other parents beforehand, so there was no way to sneak a party past them!
Good luck, I'm sure your DS loves and respects you. Peer pressure is an amazing force, I hope that working together can help your family overcome these hiccups.