My cat was diagnosed with FIP, Feline Infectious Peritonitis. Dry or no water accummulation.?

He's always vomiting and suffering from diarrhea on a weekly basis alternately. He's appetite is good but got still bony and cannot regaining his normal weight after giving foods that he likes. I feel so sorry for my cat. I hate to put him to sleep but if I can keep him as long as he can, I would do it. I need a second opinion on how can I help him inspite of the known fact that no treatment or cure for this incurable disease. He's been suffering for 6 months now and he's only 16 months old. Anyone there can give some suggestions? I gave hin all my love and care. For how long do you think he will live?