Since you don't want to stop your son's friendship, perhaps you could encourage your son to do things with other people. Have something else planned when you think the boy may want to do something with your son, or help your son find some new friends in the neighborhood, at a computer camp, or through a YMCA.
Talk with the principal at your son's school. The problem may resolve itself if the other boy has to repeat seventh grade, but in case he is promoted, ask the principal to place the two boys in different classes or on different teams. If the other boy is retained, your son may realize that being friends with him will only be a problem and the friendship may cool on its own.