It is difficult to convince our children that a certain peer is not good for them. Often times we have better luck if we focus more on the behaviors of this peer and the consequences that are likely to result from those behaviors.
The consequences should include society's response to the behaviors as well as the violation of moral beliefs and standards.
Many times we are judged by the company we keep and it does damage our reputation whether we are guilty of the same behaviors or affiliated with someone who is guilty.
Focus on qualities that make good friends and identifying reasons why your child sees these as good qualities for a friend to have. Help him identify peers that have those qualities. Encourage your child to spend more time with those peers by creating and supporting opportunities for social interaction.
You can find vast information about supporting your child, approaching the other kid parents, working with your school administration and even working with law enforcement. I hope you can find some good ideas and end this issue.
The most important thing is to support your child and make sure they know you are aware of it, listening to their feelings and reassure that you'll do anything to help resolve it.