First, WELCOME! I'm SO GLAD you found YT! There is a lot of support here
. Your little man is so adorable.
What comes to mind is either partial seizures or an ear problem. Did the neurologist talk about seizures with partial involvment? It's odd bc at times during the videos, he seems completely aware - other times, he doesn't seem completely aware - so this also makes me think of some seizure involvement. Did the neuro. discuss trying an epileptic medication to see if it would help? It may be worth a try.
An inner ear disturbance/infection/foreign object would be another thought - this would interfere with balance and could cause nauseousness - did they explore the ear pretty well?
I'm going to give this some more thought - but that's what comes to mind. My heart sure goes out to you, as this must be pretty scary to see your baby like this. Hang in there.