Hi Loki Love, I'm happy to hear your kitty is doing much better!
I'm far from any sort of cat nutrition expert, and I would be one of the last to ever recommend a vet diet, but have you considered trying an exclusive Hill's Z/D canned
diet? Then in a couple weeks when he is used to this, he is consistent and he is doing ok you can start introducing some better brands that you would prefer to feed him. Try one brand, one flavor at a time so you know if you come across one that doesn't sit well with him, and always going back to the Z/D. You could use each new food for a few days, then back to Z/D exclusively for a few days to "flush" him before trying another new one. Once you have a list of several better brands/flavors that DO work well for him you can slowly cut out the Z/D, and see how he adjusts. If you find none of the canned foods work (including the Z/D maybe?), then you can look at using slippery elm, pumpkin, or maybe even some prescription. It would be a long process of several weeks, but you should be able to get some answers by way of elimination. Maybe the vet can show you how to do enemas so you can do it yourself at the first sign of constipation?
If you do it that way you are following the vet's advice by trying the hydrolyzed protein diet, but you are still feeding a canned food. It could be that none of these foods help because it is neurological. I have heard many cat nutritionists say that any
canned food is better than any
dry food though so you shouldn't feel too guilty about feeding the Hill's Z/D canned food temporarily, right?
It may just be that he needs exclusively canned foods, and it really doesn't matter which ones. It could be that he needs the hydrolyzed protein. It could be that he needs exclusively canned foods, but is also allgeric/sensitive to some protein sources. By way of elimination you can figure this all out. That Z/D diet is essentially nothing so there is likely very little in there that could possibly bother him, so it would be like starting with a clean slate.
Just a thought since no one else suggested this tactical approach.