Is he just crying? Or is he shrieking and not recognizing that you're even there? My DD had night terrors at about 18 - 24 months (I can't recall exactly - but I know it was before my son was born). With a night terror it looks like they're awake - eyes open - but no matter what I did she didn't respond. Shrieking, screaming and crying as if a horrible creature was after her. She'd cry & scream for "mommy" - all the while I'd be holding her, stroking her little face, saying,"mommy's right here honey, mommy's here, I'm holding you sweet heart, everything's OK" - but she had no idea I was there.
It happened probably about 6 times over about 4 months I'd say. And then as suddenly as it occurred, it stopped. No idea what made it happen or what made it stop. But while it was going on it was terrifying - you can't console your child - there's nothing you can do and it seems like she's possessed or in violent physical pain, or literally scared to death. My husband thought I was exaggerating until he was awake for it and saw it with his own eyes. Then he insisted that we get her to the doctor, insisted something was wrong, etc.
It will pass - it just really spooky and weird. I prayed alot during those nights.