how to get a friend over his fear of monsters?

First, does he really believe that there are monsters there, or does he have obsessive thoughts and anxieties about them?

The two are very different things. If he really does think monsters exist and are under his bed, then he may well be suffering from delusions, and needs to see a psychiatrist.

If he knows that there are really no monsters, but cannot stop thinking about them and getting anxious about those thoughts, then that is most likely to be a neurosis. Neurotic conditions are much less serious illnesses than delusions. If it is a neurotic anxiety then while a psychologist could help, he could also try, perhaps with your help, to deal with it himself.

If it is a neurosis then several approaches might help.

He could do as Taylor suggested in his answer, and visualise himself disregarding these fears and being proud of himself for doing so, then have a go at doing that for real.

It is possible that this is connected to a need for the bond to his brother, who checks for him. He and his brother could discuss this and if it is the case what they should do about it.

It might help for him to have a thorough clear-out of his closet and to creep under his bed to sweep up there during the daytime. That would reinforce his awareness that there were no monsters there. It is important that he does this work himself, although someone else could be in the room with him at the time.

Another possibility is to install a low-output light under the bed, so that a steady glow shows from under it in the dark (so the absence of moving shadows shows there are no monsters there) and so if he looks under the bed in the dark he sees a well-lit empty space, not a dark hollow. It may be possible and useful to have a similar light inside the closet and leave the door open at night.

saw the feet off the bed....just kidding.

if he is a teenager he might need to talk to a professional because while fearing imaginary monsters is a normal part of growing up developmentally he should be past that.

try a big metal flashlight and tell him if they come for him to either shine the light which would scare them or hit the monsters with it.

tell him to face his fears and look by himself

here are two good ways i used

i used to be scared of demons cuz of this movie i watched but now i love the movies about them with these two steps

1. think about protecting a girl from the monsters and be a hero typed guy

2. just have him look on his own and then he will realize there is nothing to be afraid of

Make something that will scare him only a little but is enough to show him he can handle it that works on a lot of people

Prove to him they don't exist.

Is he autistic or something?