I don't have a teen yet, but I have two teen brothers. My youngest bro had a friend who is a bit of a bully, prone to violence and enjoys shoplifting and stealing cigarettes from his father. Since this kid was the only one who lived nearby (actually they lived next door to each other from birth until my parents moved almost two years ago) they spent a lot of time together, and we were really worried that my bro would start behaving like this kid.
My parents sat down with my bro and explained to him that perhaps X did not have the same type of consequences that would be imposed on him if they caught him at those behaviours, and that they expected more and better from him. They never said that they did not want him to hang out with this kid, but to use his own judgement.
Then one day X suggested that they pop into the cornerstore to swipe some candy bars on the way home from school. My bro did not want to do it, and he did not want to hang around in case X got caught. So he said "No, I'm not going to do it" and left. He was very worried about leaving because he had been told repeatedly that he was not to walk home alone. When my parents got home he told them that he had walked home by himself, and why. My parents were extremely proud of him, and still are. Me too.
So I guess the best you can really do is tell your child what your concerns are, what you expect of him. Let him know that you are aware that his friends are not to crazy about this new peer group either and that you hope he'll make the right decision.
Rebellion is a common thing, and the last thing you want to do is make these activities more appealing. Teens think they know everything, and they certainly know more than their parents do

Let him know that you love and support him, but that there will be consequences for behaviours that are not acceptable.
Good luck!