Hello, Brock
I am sorry your baby has gotten ill. It sounds as though your vet as diagnosed him with FIP (Feline infectious peritonitis). It is an incurable fatal disease. Please read through the link I provided you. It goes into great detail of the virus.
You of course you have the option to have another vet take blood and test it. If the second one comes back FIP positive, then you know there is no way around it.
There are other conditions that can cause a bloated stomach. Any thing that is causing bleeding in to the abdomen like bleeding ulcers, tumors, other organs, the abnormal enlargement of organs, Live disease, urine leakage from the bladder, etc.
I would get a second FIP test done. It's cheap to do.
I am sorry this has happen to you and your family. I know it's a difficult situation.
Sincerely, XXXXX
Expert in cat behavior and health
Expert in cat medical and behavior issues.
Your welcome. Please let me know if I can help with anything else.
I do hope you and your family have a safe holiday season.