I had a very similar problem with my cat - he almost died. I hated so much to think of his poor little urinary tract in pain. One thing that I had read about was cranberry and how amazing it is on the urinary tract. It had totally worked for me, but just to play it safe, I asked my vet about it. he said cranberry is safe for cats. I had some capsules that I sprinkled over his food but in retrospect i would probably get the cranberry tincture or something and put it in his water. BTW, he got completely better.
Is your cat dehydrated too? I actually ended up learning how to administer subcutaneous fluids, has your vet done this?
Pets are so wonderful and i really hope your kitty gets all better!