S.A.
There's a bedtime book by Mercer Meyer that's titled "There's Something There" Its 3 short stories about nightmares/monster/alligator in the attic/closet/under the bed. I suggest you read this with him. Because the book makes him think if nightmares/monstersin a different way!
My son is going on 3. I usually lay beside him and pat his back while he goes to sleep in his room. But he's never brought up the monster thing-yet!
Definitely monitor his TV shows. If he's in day care, ask his teacher if she knows anything about this monster problem.