J.D.
It sounds like to me that son is having night terrors. My son would get them as well when he was over tired. His doctor is the one that told me about his screaming not being nightmares but possibly night terrors. The difference being is that with night terrors he is still in deep sleep and you can't wake him up...and actually you don't want to. We did tons of research on line and was ablt to relate his night terrors to a change in his sleep pattern. Once we got him back on a regular schedule the night terrors went away...thank goodness! Best of luck!