I'm sorry that his condition has deteriorated so badly.
The "eyes wide open and nobody home" is typical of a type of epilepsy now called 'simple partial' but once called 'petite mal' or 'absences'. He loses awareness of his surroundings for a few minutes, and won't respond to you, looking vacant and 'not there'. Then he'll act oddly as he comes out of it, doing strange things (like walking in circles). He may feel tired when he properly comes out of it, or continue to have waves of more seizures in which he'll feel strange and be vaguely aware of his surroundings.
If properly diagnosed, this can be easily controlled with medication. The best med for cats is diazepam. Many anti-epilepsy drugs can't be used in cats because they eliminate them too quickly, but I've read on the internet that diazepam is safe and effective. It's used for people with this condition.
The reason I know about this is that I have a related condition: 'complex partials', which begin with a simple partial, then I lose consciousness and fall. The first thing friends notice is the "nobody home" state. Naturally, I've done lots of research related to this illness.
If you've made a definite decision to pts, then my heart goes out to you, I know how hard this can be.