A.C.
My son went through a phase when he was extremely afraid of the dark. He would scream and cry when I turned off the lights, spend any time I was in his room asking for the lights on, and if I left the room, he would throw a fit that once literally went on for an hour. He was only 2.5 at the time, and couldn't vocalize what scared him, but finally he was falling asleep with the lights on. I felt like you did (especially since several studies have found that sleeping with the lights on causes disrupted sleep and can lead to a host of problems).
My friends advised me to just CIO with him, but I wasn't willing to do so, and I would not advise it to you, either, unless you think your son is being deliberately stubborn. I never leave my child alone to be frightened on his own.
It took nearly two months, but we got him back to sleeping with just a nightlight. Here are a couple of things we did that helped:
1. Let him fall asleep with the lights on for a couple of nights, but turn them off once he's asleep to help avoid sleep disruption. If this doesn't work, he isn't ready for it yet, so let him sleep with the light on for a night or two and then try again.
2. Does he have a nightlight? If not, add one. If he does, you might consider making it brighter or even adding another in his room. We ended up buying this dim wall lamps that lit up the room effectively but weren't as glaring as his overhead. We also put a brighter bulb in his nightlight so that when we turned off the wall lights, his room was still quite bright.
3. Get him his own bedside light that he can easily turn on and off on his own. I don't know if you have an Ikea near you, but they have a very nice plastic one with a glow-in-the-dark on/off button.
I'm so sorry you're struggling with this. I hope that this helps!