From what you’ve described, I’d guess your son is probably having night terrors. Both of my children have them; my son has had them ever since he was an infant, and my four-year-old daughter started having them when she was two. Our pediatrician told us they were related to sleepwalking episodes, and sure enough my son has recently started to sleepwalk, though as a trade-off he appears to be growing out of the terrors (I’d like to trade them back, as he at least stayed in his room before, plus the screaming woke us up so we could keep an eye on him). I have no history of sleepwalking on my side of the family, but my husband’s brother was notorious for it when he was younger.
My kid’s episodes vary in both severity and frequency. The most mild attacks are pretty close to what you’ve described: sitting up and crying, often with their eyes open, but not responding to stimulation, then gradually quieting down and falling back asleep. More often is a medium episode, where they appear to be awake, cry much more violently and flail or thrash around, but are incoherent and don’t appear to have much awareness of what’s going on around them. The worst ones are when they actually get out of bed and blunder around in apparent panic while screaming hysterically. With two kids, we’ve run the gamut; my son generally just sobs and rocks back and forth in bed, whereas my daughter’s terrors have always been more along the lines of “catapult out of bed wailing and screaming for Mommy and Daddy, then ricochet around the room flailing and yelling nonsense.”
The best thing you can do when you suspect a night terror is going on is just to be in the room with the child, and if it looks like they are going to climb out of bed or hurt themselves, hold them and talk to them quietly. Don’t focus on trying to “snap them out of it,” as in my experience you’re not likely to get anywhere. Most night terrors happen in deep stage 4 non-REM sleep, so while they might resemble nightmares, they really aren’t and it’s quite difficult to wake the kid up.
That said, occasionally they do respond a bit and you can calm them down by doing the same sorts of soothing things you’d do if they were having a nightmare – rocking them, rubbing their back, speaking quietly to them, etc. If that doesn’t help, don’t stress over it. It’s more important to make sure they don’t hurt themselves (or you!) while the terror takes its course. Nine times out of ten they’ll go back to sleep and won’t remember a thing, and the tenth time they’ll wake up, blink at you and ask what you’re doing in their room. (Or ask to pee. Once or twice when Vermilad was a toddler and had just begun to have a terror I had some success in fending it off by physically taking him out of bed and putting him on the toilet, but as he got bigger this was less and less practical and it was easier to wait it out, then see if he was awake enough to go afterwards.)
You may find these go away by themselves or never amount to much more than what you’ve already experienced. The only consistent thing we ever noticed about the terrors was that both kids were much more likely to have them when they were overtired or stressed out, especially if they missed a nap. When they outgrew having daily naps, they both had night terrors more frequently for a while. We’re at the point now where we know if they’ve had a long day or they’ve been up extra-late that they’re likely to have a terror, and we’ll wait for it to be done with before going to bed or settling down, as they usually happen one or maybe two hours max after the kid falls asleep.
The worst thing about it is adrenaline-fueled wakeups and lack of sleep. My best advice is to be ready for one if you know he’s had a tough day, make sure his room is clear of obstructions at night in case he ever does try to get out of bed, and talk to your pediatrician about it if you’re particularly concerned.