Update

Kitsune most likely has FIP, which is a fatal, incurable disease. We will give him antibiotics and steroids to treat the symptoms until his immune system can no longer take it. It could be weeks or months. The vet has seen cats last up to a year. But there is no hope. The cruel thing is that it normally happens with kittens or old cats, but very rarely with a 4 yr old cat. He most likely contacted it as a kitten and it has laid dormant since then. There is no definite way to test for FIP; so it's just all signs point toward it and there you go.

Kitsune is probably the neatest, most unique cat in the world.

Yup, you did just hear the sound of our hearts breaking.