I'm sure you have gone over this with the vet, but have you checked his vision during these 'attacks'. Are his eyes dialated during these episodes? It looks like a neurological issue, but I am not a vet - just a foster mom...
At one time we fostered a kitten who had a very bad sinus infection that then entered his brain (in spite of the multiple bouts of antibiotics). He was very sick and when he had a similar episode (as your baby) he hissed and his eyes dialated. By the time we got him to the vet (about an hour later), he attacked the vet & started foaming at the mouth. We had to have him put down (the vet suspected rabies - thankfully it was not). It ended up being a 'mid-brain infection'. According the necropsy he would likely not have survived the infection and if he had, would have been greatly affected neurologically. I'm not letting you know this to scare you, but to just provide information... Your baby is NOT in as bad a shape as our little one was - and our little baby went down hill very quickly, within a couple of hours... I just wanted to pass this along in case it may help your vet rule out something or point you into another direction - NOT to scare you.
I also know someone who had a kitten with distemper who has done quite well, in spite of his disability. He held his head sideways, walked in circles and couldn't jump up on things, but has done well with his other kitten companion over the past 3 years that his humans have him!
Have you considered and inner ear issue? I'm sure all these things have been considered... I'm just throwing things out there that I have 'seen' over the years of working a foster mom... Holding their head sideways usually points me towards ear or vision issues?
I just feel so sorry for all your family - your baby, especially, and his sister, who I'm sure is witness to this and fearful and missing him at times... I wish you all well and hope that you find the answers you are looking for! Blessings to and for you all!
#PetHealthandNutrition