I think the only thing you can do is just keep telling him that its all made up and that its not real and on a night time before bed, try getting him to go to sleep in a good mood, maybe read a story or tell some funny jokes or something just to change his mood before he sleeps!

Step1 Reduce scary stimuli. Allowing your child to watch a horror movie or read a scary book at bedtime can bring on nightmares. Instead play some calming music for your child before turning out the light. Step2 Allow your child to talk about the nightmare. Encourage your child to talk to you about the bad dream and what scared him the most. Sometimes the simple act of talking about a nightmare can reduce your child's fear of it. Help your child recognize that what he experienced is common and that the scary dream characters have no ability to harm him. Step3 Eliminate any sources of caffeine. Does your child eat a lot of chocolate or drink tea? Children are more susceptible to the effects of caffeine which may manifest as insomnia or nightmares. Don't forget that certain drugstore medications can contain caffeine and contribute to nightmares in children. Step4 Consider your child's medications. Is your child taking a medication that might promote vivid dreams? A variety of medications can predispose a child to realistic dreams and nightmares. Ask your doctor if your child is taking one of these. Step5 Rule out problems at school or home. Nightmares in children can be a sign of stress. Talk to your child about any concerns he may be having about school or family. Encourage him to express his feelings. Step6 Give your child a stuffed animal at bedtime. Sleeping with a stuffed animal can give your child added security and may reduce the frequency of nightmares. http://www.ehow.com/how_2268819_stop-nightmares-children.html