May be organ failure, my cat's kidney failed and had similar things happen.
Exact thing happened to my cat. Now anytime any of our cats have a similar symptom she starts to freak out. As of now no other cat has had any health problems. Though like 3 of my one cat's teeth fell out. But he seems perfectly fine, I'm pretty sure he had cavities or something because his breath used to reek.
Why are you coming to the forums for advice. The vet already told you something was wrong with him and the only way to be sure is expensive tests. Are you asking if you should have it put down?