Help him make a Native American dream catcher (google it). Our son really believed in their powers and made a couple of dream catchers in preschool with yarn. They did wonders and he has rarely been plagued with nightmares since then (he's now 12 with MANY years of no nightmares behind him).
Also, stuffed animals and night lights can do wonders. Shadows were the scariest things for our son when he was little. A night light, or a closet light, and a bunch of stuffed animals were his best protection. And sometimes we would stay with him till he fell asleep to reassure him that he was safe.
Comfort, reassurance of your love and protection, helping him process his fears, and playful humor, go a long way to addressing the repercussions of nightmares during the daylight hours.
Best of luck!