I did not read all the responses but "monster spray" can really help. We used water scented with an essential oil like cinnamon or orange and let our kids spray wherever they want.
We have also used flashlights. Because my children are preacher's kids - even so young they have a sense of the light and darkness they hear about in church. I think it is pretty Biblical to be afraid of the darkness. Night lights in the room, the hall, and the bathroom have helped also.
Sounds like your child is intelligent for his age - so is my son. With great intelligence comes a great imagination and at an earlier age than they are emotionally mature enough to handle. I always tread lightly when it comes to what my son says he fears - he has so many thoughts that it can be difficult to get to sleep. Take the fears seriously, but do not overindulge the insecurities.
A tip for a little older child frightened by monsters:
My sister told her children that monsters can read invisible
writing and so if they take their fingers and write GO AWAY - NO MONSTERS ALLOWED on their walls that will keep them away. She has twins (4 years old at the time) they invisiblly wrote all over the house and then their monster problems disappeared. It worked pretty well with my 4 year old when she was 3. Since she could not write yet, my six year old did it for her.