Two suggestions, both of which I have tried:
First, I have raised 3 children. My daughter (now 8) and my twin cousins (now 16, I raised from 4 until 14). With the boys, I had just got them when they turned 4 but it still might work for you. They had previously been raised in a very strong Christian background. Their mother had been told she was dying and was very ill and their father was not around enough to care for them as he was either working or taking care of their mother so I took the boys in although I was only 21 at the time. Anyway, I used that strong belief in God and Jesus to help with thier fears. We utilized scripture in our prayers that specifically addressed whatever fears they were having or ones stating how powerful God is and I added how he can overcome any problem we have especially nightmares (or creepy rooms) This worked very well for the boys. It made God and Jesus even more real for them and taught them how to rely on Him when in need which is something they still carry with them today in thier everyday life. (By the way their mother did not die, she recovered fully and they are now back with their parents after being with me for 10 years, which while sad is really a wonderful thing.) The prayers did not however always work with my daughter. I know there are people who will shoot me for saying this but when it didn't work I let her sleep with me. It is only in our culture that it is not appropriate for children to sleep with their parents. The world over there are children sleeping with their parents when they get frightened or simply because there is no other bed. It doesn't harm them in any way. And if you don't make a big deal out of it it never is one. When my daughter felt she was ready she went back to sleeping in her own room by herself with no encouragement from me or her father. If he and I needed alone time we found other areas of the house, it made things interesting. As the other poster stated, this is very real for your son so continue to talk to him about it and help him manage that fear. This type of situation is where children learn to manage their emotions, skills they use for the rest of their life. So you have a great opportunity to teach here. Let him know how real God and Jesus are to you. How you rely on them when you are afraid. I hope you find some suggestions that will work for you and your family. Good luck!