If it happens the first 1-3 hours after falling asleep, it's probably Nightterrors- if it happens closer to the end of his sleeping, it's probably something else. True NTs usually happen in the early phases of sleep.
Nick has these usually when he is making major strides with a motor milestone or if he's been OT for some reason.
If it is NTs, usually the scream is very high pitched and eerie- very upsetting to hear...
HTHs....