Haven't had this problem, so don't know how well these suggestions work, just have heard that others have tried them...
1. Get him "scary animal spray" (a regular spray bottle filled with water). Go around the room before bed and let him spray in the corners and in the closet, under the bed...tell him that this keeps the animals away at night.
2. A night light (which you may have already) so that he can see that there are no animals in the room.
3. Get him a special stuffed animal and tell him that this animal will talk to the other animals and keep them away at night.
4. ASK him when he wakes up to SHOW you the bad animals...when he can't, this might convince him that there are none! I am not sure (and I don't know if you know) if he is just having a recurring nightmare or if he really thinks that there are animals in his room.
5. Does he sleep with his door opened or closed? My son does closed - maybe he can do a closed door and when you put him to bed you can tell him that the door being closed will keep the animals out - they can't open doors! Let him check the windows and closet doors as well.