Talk with your son about your family's values. Make sure that he understands that your family believes that honesty is important, and that you expect him to always tell the truth. Explain the old story of the boy who cried wolf; remind your son that people will begin to disbelieve anything said by a child who has told lies in the past. You may want to check with your public library or a local bookstore for some children's storybooks on honesty that you can read to your son.
Also, talk with the school counselor at your son's school. She may be able to give him some individual time this fall or include him in a small group. You may decide that your son needs additional help outside the school; if so, the school counselor or your pediatrician can refer you to a therapist in your community.