It could be nightmares, but it could also be night terrors. This is when a child gets up, looks and acts fully awake, but they are sound asleep. They act extremely agitated and frightened. My daughter had those frequently for a while, and now has them only randomly.
They are very common (it doesn't mean he has been abused or anything bad!) and can tend to run in families. My SIL was a sleepwalker, and my cousin had night terrors, so I guess my DD got a predisposition toward it.
Does he respond to you, or does he look right through you? When you try to comfort him, does it just wind him up more? Those are 2 major signs of night terrors. They also don't remember anything that happened that next morning (while you are still freaking out)!
Next time, try hanging back and not interacting -- this is HARD to do, since our first instinct is to rush in and comfort our children! Just watch and make sure that he doesn't hurt himself, and if he's out of bed you can guide him gently back at some point. If dealing with things this way tends to help, then you're most likely dealing with the night terrors.
Once you realize what the best way is to deal with him, and you are confident in how to deal, it will get better, I promise!!