When your child becomes histerical in his sleep, is he difficult to wake up? If so, you may be dealing with the same thing I did when my oldest daughter was his age.
A friend described these fits as "night tremors"
I don't know if that's a technical term, but the name seems to fit.
It can happen to children who don't feel secure in their environment. When my daughter began this very scary behavior, her father and I had seperated, and she and I had moved around a lot to different friends houses. I was working a lot to support her, so I wasn't around as much as I used to be. I belive these series of events to be what triggered her nightmares. It was really scary for me because I would turn the light on, and call her name, I'd put a cool washcloth on her forehead, I'd try to hold her and comfort her and she would just scream and scream, not waking up.
Now, maybe nothing that extreme has occured in your household, but if your son just started going to daycare, or maybe he moved out of your room into his own, maybe a family member has passed away and you're really upset about it, it could effect him, even if he doen't understand what's going on.
Don't worry. This won't last forever. Just keep up a good routine and let him know how loved he is, try to not stres out with him around, and things will get better rather quickly.
I hope my advice was helpful to you!