T.G.
THis is a very real fear for kids. I remember being afraid of the dark as a child, so I am ultra sensitive to my daughter who is 5 and recently claims to be afraid.
Some suggstions would be to leave his bedroom light and or closet light on. You can also purchase a childrens flashlight or plastic lantern. The hall light in our house also lights up our bedroom, but if we are having a rough night with her, I will leave the light on and shut my door almost all the way. If thats still too bright I will cover my eyes with something.
To me, I would rather get a good nights sleep, so leaving all the lights on is worth it to me.
During the day, play a shadow game. Use a flashlight to make shadows on the wall and ask him to guess what they are. Then, graduate to guessing shadows on his bedroom wall, using his light. Ask him, is that the shadow from your table, is that the shadow from your toy box, etc. You should eventually be able to graduate to playing the shadow game at night, and then eventually the light situation can go back to normal.
Also, maybe he has seen a cartoon recently that has scared him or maybe an older kid told him a scary story.