I can't speak from personal experience because I haven't had to deal with that issue.
If he is truly scared of monsters (and not just making it up so he can have his way) - you could try Monster Spray. I've heard of other parents doing this. Fill a spray bottle with water and mark it "Monster Spray" or whatever. Tell him it is a special spray that monsters hate so it keeps them away. Every night you 2 can spray in the closet, under the bed, and anywhere else he's scared the monsters are lurking. Keep it by his bed - so if he wakes up in the night he has it.
I've heard of this working for other people, but as I was writing it, it made me question if this was the best idea. It is just confirming that there are monsters. However, if you've tried explaining there's no such thing as monsters, and he's still scared, it may be worth a try.
OMG - I had forgotten all about this until just now - when by daughter was about 4, she was scared of monsters for a short time. I told her that it was Mike & Sulley (from Monsters, Inc.) that were in her room- & they would chase away the bad monsters like Randall. That worked for her.