What could cause a child to scream and cry during his sleep? How to help stop it?

It has a lot of reasons. It could be something about his physical state when sleeping--he isn't in a comfortable position or his diaper is irritating him. It may very well be he is having a dream that he feels uncomfortable, too. I suggest that whenever you put your child to sleep, make sure he is very comfortable, area is well ventilated and you can also put some calming background music.