You may also want to leave a nightlight on in your son's room, or a light on in the hall. If he wakes up during the night, he'll be able to get his bearings and see that he is safe. As his sleeping improves, you can reduce the light gradually until it is the way it was before the bad dreams began.
If the problem worsens, or goes on for a period of weeks, you might consider finding a therapist to work with your son. The school counselor or your pediatrician can refer you to someone in your community.