If this was a sudden development and he had never spoken of monsters before I'd be more worried about the possibility that there is actually something threatening your son, but if his imagination has always generated monsters I'd be inclined to suspect the threat is in his imagination.
"Monsters" can become very real to kids, and once that reality takes hold in their heads it can grow. You need to find something that makes your son feel safe, and make sure he takes it to bed with him. Imaginary monsters can be fought with the imagination. A friend of mine had a son who had a similar situation. He had nightmares and was convinced that monsters came into his room at night. I bought him a "monster zapper" gun that I saw in a toy store and gave it to him, without any real hope that it would help. Apparently that worked really well. Because it would light up when the monster was zapped it made the shadows disappear and so the monster would seem to disappear. He kept score on a white board of how many monsters he had killed and he actually started looking forward to bedtime.
You might try a soft lights in different colors. I found pink was a really good color for my daughter; it made her room seem "warmer". My son, who also suffers from nightmares and anxieties, goes to sleep with soft music. (I personally like Enya for soothing, but he finds pop more comforting. Go figure)
The thing is that you should accept his fears (in a low-key way) and use psychological judo on them.
...okay, if monsters are real then so is the power of this monster zapper
...did you know that monsters can't stand the color pink? If your room is full of pink light they don't dare come out.
At this point he "knows" that monsters are real. If you deny it, his fears will overcome your authority factor and he will conclude that you just don't know about the monsters. If you engage with his imagination, then he will believe you know what you are talking about when you tell him that pink light makes monsters dissolve.
Here are a few cute ideas: http://www.wikihow.com/Eliminate-Monsters-Under-the-Bed, although I could see the "monster spray" getting out of hand.
Parents.com suggests that if your child is afraid of something, you introduce a friendly version. http://www.parents.com/kids/development/behavioral/understanding-kid-fears/ Has he seen Monsters, Inc? If not, sit down and watch it with him. Discuss it with him, relating to the monsters that he has in his room. You might get him a large Sully doll or poster to ride herd on the other monsters. Once Sully takes up residence, Randall and his buddies won't be allowed in the room.
Don't worry about him feeling that you have "deceived" him, he'll figure it out when he grows up. If he even remembers. The only way he'll feel you have lied to him is if you turn on the pink light and the monsters really do come out from under the bed...and then you've both got bigger problems than a little pink lie.
