PLEASE!!! Do NOT end his life. FIP is not a death sentence. His testing positive simply means that he was exposed to it. It doesn't mean he has it.

My cat MooShoo tested positive for FIP when he was 10 weeks old. He's now a fat, healthy 6 year old Sphynx. I also have 3 other cats that live, eat, sleep and play with MooShoo. They're all healthy.

The best thing you can do for him is have him neutered and let him back out with his pals.