D.P.
I can't believe you have not tried Monster Spray! Use a plant mister or the like and help him "spray" his room before he goes to bed. That night help. You can even leave the spray with him just in case he needs it. It might be fun to yell "all monsters OUT of this room!" as you are spraying.
On a side note, my son actually HAS a monster that lives under his bed. Monster started out as a pile of shoes, boots and gloves...now he is completely contained in an under-the-bed storage box. He's a part of the family. But, apparently, he's not a scary, bad monster.