A bad company often tends to have a very bad influence on his life. You could take instances showing how he has transformed from who he was to who he has become now. You could let him involve in stuffs he is interested in so that he slowly tends to come out of the company which has a bad influence on his life. Moreover, it is most important, if he understands the consquences he might possibly face. Only if he understands would he come out of that company.
Hope it helps.