I agree with M2M.
Could be anything from growing pain, to teething, to wanting to eat extra due to a growth spurt that has him up crying. I agree with the thought of trying some pain reliever, or maybe some Hyland's teething tablets. They're all natural and whether he's teething or not they have great stuff in them for sleeplessness, restlessness, and irritibility etc.
My son has had a few night terrors in his little life, and actually I am thankful to say that he has not had one in ages. Not since he was under two I want to say. Let me tell you, you don't have to be educated to be able to tell the difference between regular crying and a night terror. It is one of the scariest things I've ever seen my son do. I would hear him wake up crying in the middle of the night, usually shortly after going to sleep (which is the time when night terrors happen, not usually later in the sleep cycle) and he would just scream this horrible scream. My husband and I would go running in there and usually, if it was anything else, I could pick him up and comfort him and get him what he wants and he would be fine and calm down. But the few times it was a night terror he would just continue to scream and scream and nothing I could do could snap him out of it or calm him down.
Its scary because there is absolutely nothing you can do but wait for it to end. Its like they can't tell you're there.
I'd have to say that the only thing that sticks out in my mind about what triggered them was that he usually missed his nap on those days. I think it can be caused by messing up the sleep pattern, but I'm not sure. I would just make sure he gets the sleep he is supposed to and it should be fine. Also, only some kids ever even experience night terrors, not all. So there's really only some chance that he'll ever have them.
But if you ever do have to go through a night terror with him, you will know its happening trust me!