Almost every child has gone through a phase of being afraid of monsters. They see them in the shadows of their room while lying in bed, they think they're in the closet and under the bed. It can be very exhausting for a parent to constantly reassure their child that there are no monsters in their room.
I have an easy, inexpensive suggestion. Buy a can of air freshener, preferably lavender scented as it can have a calming effect. Make a label that reads, MONSTER SPRAY; wrap it around the can and secure it with glue or tape.
During your child's bedtime routine (your child DOES have one I hope) spray the room while your child repeats the phrase, "I'M NOT AFRAID!." It should help ward off his fear and make bedtime much easier.