Thanks for the replies! I'll try to answer all of the questions and then describe last night, which was horrible!
It seems that he can move during his episode. During the few seconds that he's stiff as a board, it's hard to tell if he would be able to move or not because he's purposely (so it seems) stiffening up. I wouldn't be surprised if a doctor told me that portion of it was a mild seizure where he couldn't speak or move, but if so, it would be a very mild and short (again, a few seconds) seizure.
It does not stop after he pees. In fact, getting him out of bed and not holding him seems to upset him more.
He doesn't try to find his mommy once I'm there. Last night I tested this within a few seconds of going into his room while he was crying. I said, "Look at momma," and he immediately turned and made normal eye contact with me. He was able to talk. He even asked (in screaming tears) to go to the bathroom. However, after he went, he still couldn't stop crying for another 20 or 30 mins.
He is not scared of me leaving or just letting him cry. He's a perfect angel when going to sleep - no sleep associations needed to get to sleep. In the middle of the night when he stops his crying fit, he settles down and really just wants me to leave so he can sleep. I've tried to ask why he was crying or if something hurt, and he always just says stuff like, "nothing." I kiss him and tell him I'm going back to my bed. He says ok and I leave. So I truly don't think it's a fear of me not being there, leaving him, attachment issues, etc.
He is not on reflux medicine anymore - hasn't been since he was about 11 months old. His pediatrician says she can rule out any dietary issues (intolerances or allergies) and reflux issues because he has weeks and even a month at times where he sleeps perfectly all night every night, without diet changes.
I think I'm going to treat them as nightmares for the time being, but I still am not convinced. We did have a chat this morning about how our mind "thinks" and "tells stories" while we are sleeping, and big brother (10) was able to "confirm" that his mind thinks at night too. My LO didn't seem to be listening or too interested in what we were saying. Hopefully he was peripherally listening!
Last night, he was asleep by 8:30pm. Woke crying at 10pm, settled fairly easily (a few mins). Same thing at 11:30pm. 1:45am he woke crying and wouldn't settle well at all. This is when he asked to go to the bathroom. I did have him roll over to his back and that seemed to keep the stiffening from happening (or could have been a coincidence). He was fully awake and trying to talk with me. Some things I understood ("I need to go potty," "I want bunny," "Where's my blanket," etc.) but much of it was through so much crying that I couldn't understand. I cuddled him in bed, I tried to soothe him with music, I walked with him around the house, I rocked with him in the chair. Nothing seemed to calm him down. Finally after 30-45 mins of crying, I had put him back in bed, he asked for water (which we keep in his bed and I had tried to offer to him a few other times during this episode), then he cuddled into his blanket and was ready to go back to sleep.
My current thoughts (though everytime I have a hypothisis, that night's episode proves me wrong) are that he is having the full bladder feeling, causing stirring and light sleep, causing nightmares, and then waking without being able to calm down. I do agree with the fact that I'm probably working him up more when I ask all about what's wrong, does something hurt, etc. I don't do it in an anxious or panicky way, but maybe I'll try to just go in and pat his back and repeat the same few phrases over and over softly. (Though as I say this, I'm reminded that when I try to sing soft songs to him, it just ticks him off more!) Maybe I'll also try to cut off water (we've already stopped milk past 3pm) past dinner time to keep his bladder more empty and remove his water from his bed (it was put there by suggestion of the pediatrician who thinks he has no sleep issues).
Any other thoughts?? I'm so desperate for help! I work full time and can hardly function on 4-6 hours of sleep nightly, compounded for 3 years! I do get a few nights here and there that are good long 8 hour stretches, but they are never even close to enough for me to catch up.