I am currently battling the same type of symptoms with my 3 yo. He currently was diagnosed with absence seizures and is on medication. We are going tomorrow to have a sleep deprieved eeg. Were you able to find out if anger was a trigger? I only ask because I think that is something that I am noticing with my son.
I would definitely see a neurologist and get an EEG done to check for seizure activity. If possible, you might want to have the tech doing the EEG do something that would make your son angry. I know it sounds a little strange that we would want to make him angry. However, before I had surgery for my seizures, they hooked me up for an EEG (Mine was an internal one that lasted a week but his would be external and take an hour or two.) and then exposed me to my 'triggers' so I would have a seizure and they could see exactly where they started, went to, etc. If he gets angry during the test, he is likely to have this paralysis stuff happen and they can see what is going on in his brain when he does that.
It does sound more like a seizure.
Talk to his doctor. Might be some kind of seizure.