I am very sorry about your cat.
Unfortunately, It does sound like he has the wet or effusive form of FIP. Since your cat is so young he probably had the FIP since birth. I would contact the breeder about the disease. The breeder may not even be aware that the disease is present in her breeding facility.
The following is a list of Non-FIP diseases that can resemble the symptoms of FIP. There is a possibility, no matter how small, that your cat does not have FIP but has one of the following instead: (The following information is from the "Merck Veterinary Manual" an important veterinary text and reference book)
1. Ascites (free fluid in the abdomen) due to congestive heart failure
2. Ascites due to hypoproteinemia (from renal and liver disease, glomerulonephritis, malabsorption, or parasitism),
3. neoplasia,
4. bacterial peritonitis,
5. toxoplasmosis.
These other diseases can be ruled out by blood tests, X-Rays, etc.
justine. g @ u k y . e d u