Consult with a doctor to diagnose any potential medical causes. Some speech impediments arise from a hearing impairment or swollen adenoids.
Have your child silently practice forming the letter 'S' by having him thrust his tongue toward the back of his teeth around the palate.
Have him use a mirror to monitor the formation of the mouth, tongue and teeth. He should have his tongue properly in place while projecting air outward to pronounce the 'S' sound correctly.
Enroll the child in speech therapy conducted by a professional speech therapist who specializes in lisps and other 'S'-related speech impediments.
Encourage the child to practice at home and praise and reward his efforts to motivate further progress.