I don't know when it started, but I also developed a horrible fear of the dark as a child. If the light burnt out in the hall at the top of the stairs, I would be absolutely terrified to go up the stairs alone. (I was actually still scared even if someone came with me, but at least with someone else there, I could do it, and not just be stuck at the bottom of the stairs, unable to make myself move).
I want to echo what a few other moms have said: validate his fear. He really IS scared. It seems like being afraid of the dark is silly, but it can just be truly frightening for some kids.
I felt like if my family had actually understood that I was terrified, I might have felt a little better.
I developed my own solution: singing. Whenever I had to be somewhere dark, I sang or hummed, usually out loud, but sometimes just in my head. I picked happy boisterous tunes. You may be able to teach your son to sing a song when he's scared of the dark. Doesn't matter what song, just so he is concentrating on something other than the darkness.
Also, ask him what about the dark is the scariest, and ask him what would help make him less scared of the dark. He may come up with his own solution.