Too scared to sleep!

#5 who is only three years old has a habit of staying up late. I think he got used to it from when he was really little and if he wasn't tired yet we just let him stay up with us while we watched a movie or whatever. Now he feels like that is his personal time with Mom and Dad, and as soon as the other boys go to bed he is asking about watching something together. Usually he falls asleep very quickly and we carry him up and put him in his bed.
Yesterday evening he was telling me that he was very sleepy. It was near bed time, so I said he should go to sleep if he was tired. He said that going to sleep was scary because he is afraid of the dark.
I did my best to comfort him, and we did a little experiment with looking around the room with the lights on, and then turning the light off and feeling our way around to show that the same things are there in the dark. I thought that would help him, but I noticed that he still kept himself awake until almost midnight. Hopefully it was just because he was enjoying time with Mom and Dad, and not because he was too scared to sleep.
(Note: He usually falls asleep within a very short time after the other boys are upstairs, sometimes even getting to sleep before they do. The midnight thing is rare.)