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Hi this is Dr. Hill. Unfortunately, what you are describing sounds like Feline Infectious Peritonitis, which is an incurable disease. The other possibilities are peritonitis (inflammation of the lining of the abdominal cavity) secondary to a perforation of the bowel (typically a foreign body) or cancer in the abdominal cavity. The only way to know is to take him to a veterinarian. If he is suffering, then humane euthanasia may be the best choice. Kind of you to take him in. Please post any follow up questions. You can post on this thread for a couple of days, even after rating my response. Take care.