If you feel that you can make a difference in the other boy's life, give it a try. Invite him to your home to spend time with you, or include him in a family outing. You could possibly be the positive influence that he needs.
You may believe that this is not your role and that your son should stop being friends with the other boy. If so, talk with the school counselor about your concerns. He may be able to work with the boy and his family to help them make some improvements in his situation.