L.S.
I have a family friend who had her husband and son plan a "monster hunt." They put on camoflauge and her husband got a big trash bag (empty). He had their son stand quite a ways away from the closet while he went in with a flashlight and had a mock struggle. When he came out, the trash bag was stuffed with pillows and blankets, and he hauled it out, pretending the monster was in the bag. Then he took it to the front door, threw it out in the yard and yelled "And STAY OUT!!" and slammed the door behind him. My friend's son is in his 30's now and was never scared of monsters again! Might work, might not, but it would be so fun, right? :) If nothing else, you could watch Monsters, Inc. with him and see if that quells any fears. Good luck!