Then my first thought is to try San Ren Tang (Three Seeds Decoction). It is one of my favorites for inappetance in dogs, iguanas, cats, etc...
It should also reduce the tendency to cystitis. It can even reduce low back pain (lumbo-sacral region), especially where due to nerve root compression from disk disease.
For diets, I favor the 'Paleolithic Diet' from the Manual of Natural Veterinary Medicine, if you have that book (Mosby, 2003).
Good luck!
Steve