It could be nightmares, but I would wager that it is more likely night terrors. You child seems awake, but doesn't respond to the usual soothers. They usually occur within a few hours of going to sleep--they occur in a lighter stage of sleep than dreams and nightmares. My daughter would have to be taken outside to get get calmed down.
He is at a classic age for developing night terrors. My daughter has had bouts of them on and off since just before she turned two. I have figured out most of her triggers--changes in schedule/routine (like when we changed her daycare); some medications (singulair, zyrtec, some cough medicines); and ear infections.
One of the best things you can do is keep a journal of the times when he is waking. Sometimes you will notice a very distinct pattern...waking him just a bit before you expect one can help avert the episode.
Once she is in an episode, it works best for us to let it pass without trying to wake her.