Is your cat an indoor only cat or allowed outside?
What kind of food is he usually fed?
Do you have other pets?
You're doing all the right things with this lucky cat - I hope his fight to make it through whatever this turns out to be is successful.
Obviously nothing beats a hands on exam and treatment, exactly like you're doing, so what I'll do is offer some of the more commonly seen things I've found over the years (I own a rescue org - and of course cats are our biggest customers).
When a cat stumbles, takes a few steps and falls over or generally seems to be off balance, a number of causes may be involved - I would expect that your vet is doing blood testing for various things such as infection, virus and so on.
When we know that a cat has a questionable background with nutrition, such as a previously feral cat (many of our rescues are born to abandoned cats and have never had good nutrition) we see this symptom as the result of deficiencies.
In other cats this can be the result of an inner ear infection or even a tumor effecting the brain stem, cerebellar regions or possibly feline ischemic neuropathy. This is sometimes a symptom related to Feline Leukemia Virus, Feline Infectious Peritonitis, etc. which are diseases that may cause central nervous system symptoms such as this.
Feline Vestibular Disorder is generally accompanied by rapid/or otherwise abnormal eye movements and is usually pretty sudden onset. With or without treatment it tends to self-resolve in less than a month. Obviously it might be something more serious (tumors, viral infections/bacterial sources, even liver problems) .
Modifying your cat's diet to better follow feline nutritional needs is important and how to do this may be surprising to you. Unfortunately, many vets don't have the background they should have when it comes to advising on feline nutrition (even if they're selling a designer brand food).
A vet who is specialized in feline nutritional needs has become determined to provide everyone the correct information without ulterior motives (no sales), including recipes for homemade options and comparisons/evaluations of cat foods - take a look here http://www.catinfo.org/
Finally, it must also be considered that the cat is suffering low blood sugar/hypoglycaemia, a blood clot or may be epileptic (though less likely). Don't you hope it's this simple? I had one little girl very recently who spent a week in the hospital for what looked like failure to thrive and after some IV fluids and balancing her electrolytes and blood sugar - she bounced right back.
The dilated pupils and rest of your description however is of much more concern.
There is also the possibility that the cat has been exposed to a toxin. It may not show up in the screening your vet has done, but treating the symptoms (as your vet is) is prudent and the right thing to do.
Finally, suggest to your vet that they measure erythrocyte transketolase activation after the addition of thiamin pyrophosphate is the standard (biochemical) test for thiamin deficiency. Some clinicians who see a large number of feral cats will know upon presentation that the cat is nutritionally deficient.
Neuro-anatomical changes when a feline suffers a B-1 deficient diet might include seizure activity, ataxia, gait disturbances and even mental retardation. Some kittens born to a deficient mother will show symptoms from the time they can walk. Often, injections or supplements of B-complex, B-1/thiamin will lead to almost immediate results (within 24 hours in many cases).
Cats who are frequently fed fish or seafood might suffer thiamine deficiency, or cats eating dog food.
Who knows what this sweetie got into ?
I hope he recovers quickly and is home soon.