8 year old male with ataxia (wobbly back legs)

I am distraught. I took my 8 year old male cat (Bon-sai) to our vet two years
ago for what I perceived to be a neurological problem. I noticed that he had
become clumsy, and the vet told me that he was obese and that that was the
problem. I was a bad pet owner and didn't get him to lose the weight. We have
5 indoor cats, this cat is a furry vulture (a very loveable one though...lol),
and it's very hard to meal feed all of them.

Note: all my cats have been fed an extremely well-formulated cat food (Flint
River Ranch) since they were kittens. This food is $40/20 lb bag and is very
low in ash and magnesium. All ingredients are human grade.

I should have trusted my instincts. The clumsiness problem has gradually
continued to get worse, and I took him back to the vet today, just to put my
mind at ease and also because he's been defecating in very bad places, like
on our bed. He's always pooped outside his box in the garage, and we've tried
every remedy imaginable (every litter on the market, including CatAttact,
cleaning the boxes 2x/day, cutting down the side of the box for easier access,
etc.)--we figured that he was just a finicky cat who didn't like the fact
that other cats used the boxes (we have one box for each cat). Actually, I
think that deep down I was terrified that the vet would tell me that he had
something like a brain tumor and he'd have to be put down. She finally has
admitted that I was right and that he DOES have some kind of neurological
problem. He has something called "head tilt" which I had never noticed before
(he walks with his head tilted to one side). She said that he also has a
heart murmur, but she didn't tell me to what degree.

Diabetic neuropathy? Maybe, but not likely. He doesn't walk on his heels,
he's overweight (not underweight), there's no excessive thirst, and his coat
is as soft as a mink's. That rules out thyroid problems too, right?

Brain stem or vestibular tumor? I thought of those too, but he doesn't have
any nystagmus at all--wouldn't he have a nystagmus if he had a tumor? The
only symptoms he has (besides the fact that he weighs 15 lbs and he's a
pretty long cat) are clumsiness in his back legs, inability to pivot,
reluctance to sit on his hind quarters (he usually lays down on his side),
and I've noticed that when he DOES sit that he kind of sways back and forth
when he purrs.

Any ideas? I'll get the results of his blood workup tomorrow.