Does he have a little night light?
My ds always slept in pitch black, blackout curtains and door shut but things did change when he started seeing things that he could be scared of, imagination kicks in & then he would be like your son. We got him a gro clock (also to help EW's & the light from the screen) and a very dim night light, and had to leave the door slightly open. I found the decision really difficult actually as i struggled with iy being a 'phase' and to ride it out but it didnt effect his night sleep what so ever, which i thought it would with being used to sleeping in the dark. I think DS was closer to 3 though at this point.
DD went through a 'dont turn light off' stage about 3/4 months ago (she was 2 in march) so we got her a gro clock & she sleeps perfectly fine with that dim light. Her door is closed though. I guess it gives them a sense of security. (They also use the gro clock so they know what time its ok to 'get up' or shout us, im a big fan of gro clock can you tell ha)
A dim night light is something to consider if you dont already have one. Also, ask him what would make him feel less scared? He would probably say he wants you there but you can then explain 'mummy has to go to bed too, but how about a little light? Would it help if we leave your door open a little instead of mummy sitting with you?'
Sometimes i get scared in the dark if ive watched a scary movie & only feel comfortable with my hubby next to me, and im an adult so i cant imagine how lo's feel when they have a genuine fear!