Is he crying or is he hysterically screaming? When he it's happening, can you comfort him? Does he seem to still be asleep, not really there or is he awake & aware of his surroundings?

Night Terrors as compared to nightmares & other things are characterised by the child not being able to respond to their surrounds. Basically until the terror is over, they are stuck in it & no amount of comforting will help them, it may even make them worse.