When mystery illnesses strike I often think of immune suppression of some sort. There are other causes than FIV to have an inmmune system compromised. It's just a thought though. Your theory of an infection affecting the brain such as meningitis sounds plausible too. I'll be including Diablo in my thoughts and daily requests.
The suggestions to feed him posted are really good. Sugar Cat Mom is really good with nutrition knowledge so you could sure use her advice.
It's really hard not knowing, not having a diagnosis. It's one of the worst parts. In the mean time vet bills growing plus the pain of seeing your cat not well.
I would also try a holistic veterinarian. He or she might have a different approach. Although seems that your vet is getting on the right track with the nervous system infection.
Go one day at a time so you won't be way too overwhelmed and keep Diablo eating and hydrated even if it's only from the water that food contains.
Get better Diablo. Get well and recover your strenght.