Some of the causes are severe liver disease, right-sided heart failure,
hypoalbuminemia (a decrease in the blood albumin level), an abdominal mass, bleeding disorders, lymphatic diseases or an obstruction of the caudal vena cava or hepatic vein compromising blood circulation
Treatment, of course, depends on the cause. Until a definitive diagnosis is given, try to keep him as calm as possible because the fluid retention can make breathing difficult. Please keep us updated on his condition.