Monsters under the bed...

A few weeks ago Matthew had a terrifying nightmare. From what I can gather, a monster came out of the drain in the bathtub and took him back down it. Since that night, the fear of this monster consumes him. He won't listen to, "There are no such things as monsters." He KNOWS the monster exists and won't listen otherwise. I don't know how to explain to a 3 year-old that monsters don't exist on a level he can understand. One of his favorite songs is Heavenly Father Loves Me and so using this song I tried to teach him that Heavenly Father wouldn't create monsters, only beautiful things. No go. We have to have lights on at all times of the day, all doors, inside and out, must be closed and locked at night, every sound is scrutinized and MUST be identified or a breakdown is soon to follow, he sleeps in our bed everynight (which isn't a new thing) and I have to have my arm draped over him all night, and I have to go into the bathroom with him every single time he goes. Most kids are scared at night, he fears this monster 24 hours a day. If there is anyone that has any suggestions, I am happily awaiting them. As I am writing this, I am being scolded for turning off my bathroom light. Oops.