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There are a few techniques that may help. Incidentally melatonin helps people to get to sleep but does not help with nightmares. He is having the same feelings I have been having with the revival of the "walking dead" in TV and videos. It is scary for no reason. So the best way to deal with this is to create a story with him each night. You might start with, " a boy is having a scary nightmare." you take turns making the story and your job is to get
the boy in the story to make the scary people go away. Your son will catch on and this type of positive imaging will help in his having better or at least less scary dreams.