C.F.
With all my years with my own three and teaching preschoolers, here is my two cents worth. Hope it helps.
I think you will need to observe closely, difficult I know when he are crying or screaming, to figure out why. At this age it could be seperation fears, bad dreams, or awareness that crying or screaming gets a reaction or result.
At this age, it could be awareness that life goes on even when they are asleep. Mom and dad are doing things and he might feel as if he is missing out.
Children do not like to say good-bye, it makes them sad even if they realize you are coming back. Maybe good night makes him sad.
At this age, children also may begin to become more aware of dreams. Or maybe something scared him one night. TV and movies sometimes cause this, even if you are careful of what he watches.
And, sometimes it can be that it gets a reaction that they desire. If he can turn it off quickly if given the result he wants, then you know.
If you are sure he is truly upset, you might try sitting on the floor next to his bed or in the doorway. Don't interact very much but a quiet song or just a simple shh, it's okay, shh...might reassure him.
Another thing to keep in mind is at this age, chilren often need to go to bed much earlier than when younger as they are not taking so many naps during the day and they are growing very quickly both mentally and physically. Sometimes when children are very tired, they can become upset much faster and to a higher degree.
And, just thought of this, if in a growth spurt,hunger could contribute to this. At times, I have given mine oatmeal to eat before bedtime.
If I think of any other suggestions, I'll pass them on.