These are some great responces, My son went through that and I told him that everything will be ok. I told him that he needs to scare the monseter away, so I tought him how to scare them away (that gave him some control) then I also tolds him "Monsters are more scared of you then you are of them, but do you know what those monsters are really scared of? Mommies and Daddies. They will not even come near our house because we are here." That seemed to help. My friend filled a squirt bottle with water labled Monster Repelant and let him squirt the repelant around his room.
Just read your sons emotinal needs, mine need hugs and mommy so I incorperated that in my monster explination, also I showed him the movie Monsters Inc where they are scared of kids more than anything else, but they have a job to do, so he got it and we have not had anymore monster issues. Just do not tell him that they do are not real and there is nothing to be afraid of, becaus right then and there he is afraid and he will not trust you if he cannot come to you because you will not believe him that they are there.
Good luck.