In my experience it could be a nightmare or maybe a night terror. My dd (2.5) sometimes has nightmares and has had a night terror once. That's when they look as if they're awake but really they're not, their eyes are open and they scream, you just have to leave them to it. If it's a night terror, just be there, make sure he's safe, don't try to wake him. It's a very distressing thing to witness, but nothing you can do about it but wait for it to pass. HTH