A.G.
It will probably be a bad couple of nights but you just have to be tough in the new rule that he has to stay in is own bed. Like the other responses it's good to give him something to keep him "safe" from the monsters. I used to give my daughter a flash light. Let him listen to soft music or like my sister who's son was very scared so she just let him sleep with his light on for a few months. Kids go through little phases like this often so you just got to do whatever works. But after you give him something to calm him down don't let him in your bed. Even if that means placing him back in his bed over and over again. Once he realizes he won't get his way he will stop. I had to do this with my daughter. It took only one night but I had to keep putting her back in her bed for an hour straight!