Feline Infectious Peritonitis (FIP) is an uncommon, but usually fatal, aberrant immune response to infection with feline coronavirus (CoV), says Wikipedia’s entry on the disease. It says that the virus (CoV) is “common in places where large groups of cats are housed together indoors (eg., breeding catteries, animal shelters, etc.). The virus is shed in feces and cats become infected by ingesting or inhaling the virus, usually by sharing cat litter trays, or by the use of contaminated litter scoops..”-
– I hope there aren’t other cats around Charlie, that he has his own litter box, so that other cats don’t get infected.
I didn’t read all of Wikipedia’s entry, but if you didn’t read it, maybe there is something there for you to use so to help Charlie somehow.
“I just want this to not hurt”- if only we could accomplish that, to not feel hurt, that is, emotional pain. Be kind to yourself and to Charlie, best you can.