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Children's fear of darkness is very common and they usually grow out of it.
Have you tried putting soothing children's lamp or nightlight in his room?
Can you stay in bed with him for awhile while he falls asleep in his room? He feels a bit insecure and has a bit of separation anxiety and needs a bit more comfort from you to help him relax and fall asleep.
Here is an app that can help you.
It is a nighttime story and there may be a nightlight available that goes with it.
Another good trick is to take him for a walk in his room IN THE DARK to show him there is nothing to fear about darkness.
You might try this night light globe projector the can have tropical fish swimming on the wall - very soothing:
or even better, this starry night projector with lullabies and soothing sounds.
This is a time where you will have to invest some energy, as well, until he overcomes his fear. He will.
Elliott, MAE, LPCC, NCC, CCMHC