S.H.
At this age or even younger, developmentally, a child develops general "fears." And yes of night time and the dark.
It happens.
I even remember my childhood like that.
It is their cognition and imagination... that is changing. As well as their developmental impressions of the world.
If this helps, keep in mind that at this young age, a child does not know intrinsically, the differences between fiction and non-fiction.
They do get scared. Even if to us adults, it is nonsensical.
It is an age phase.
But even my 9 year old daughter, sometimes gets afraid of the dark and nightmares.
Normal.
If you try to convince him that he is wrong, it will not work.
They do really feel, that it is real and frightening.
Think back to your childhood as a young child. Things like that really do tweak a child's imagination.
For us, when our kids are sick or really frightened, we have a floor futon that they can sleep on in our room, on the floor. It is temporary. They know that. We know that. It works.
Or, you just sit in a chair with your son, until he falls asleep.
My 9 year old daughter, once had a bad dream. Then she didn't even want to sleep IN her bed because she said it made her have a nightmare. So she slept on the floor of her room. Fine. Then she got over it. Fine.
Since his siblings do not like sharing a room (with him) do not put him in there. It is not their responsibility.