Hello...Thank You for using Just Answer. This is a very common fear for children to have. What you can do is to get a cheap net or some type of basket and tell your son that you are going to put all of his fears about monsters away in that net and then actually throw the net away. Explain to him that all of his fears are gone. You have already awknowledged his fears so you don't want to harp on them any longer than needed. If he brings up monsters again, calmly and positively remind him that all of his fears were disposed of.
Another problem may be that he has heard the story, There's a Nighmare in My Closet which discusses how a young boy has to deal with a monster in his room. It is a kid book but can be frightening. You may want to ask the teacher has she mentioned monsters to the class and explain your concerns.
Also, make sure that he is not watching tv with images of monsters.
Hope this helps!!! If not, just ask more questions.