Think with their mind:
You should come at their level and tell them that you completely understand their condition. Give them examples from your own childhood just to make clear that nightmares are bad dreams. All those monsters and fears do not exist in real world. Tell them how you overpowered those fears in childhood.
Inject positive thoughts:
Fill their minds with positive thoughts and images for example; ask them what the best thing of the day was? What they want to see in dreams?
Talk about their favourite super hero who always rescues others in the time of worry. This will relax their nerves and lessen the probability of nightmares.
Turn on the night bulb:
Children are always afraid of darkness and if you are leaving your kid alone in a dark room what else do you expect than a bad dream? Always keep the door open and turn on the light so that he feels secure while going for sleep.
A sleeping partner:
At least give him an imaginary friend who will protect him during nights. It can be anything like a brave lion or teddy bear. Ask your child what kind of guard he wants and then put that next to his bed.
Give him a quick response after he gets disturbed due to nightmare. Make him feel safe and secure by tucking him into bed.