Your dad is basically starting to have symptoms of liver failure. Probably, most of his liver has been replaced by cancer and whatever little is remaining is not sufficient to sustain normal body functions. When the liver fails, it leads to accumulation of fluid in the abdomen and lower limbs. Simple remedies can help alleviate his symptoms partly. Taking a low protein diet is very important as any proteins that we take are metabolized by the liver, putting more stress on the already failing liver. If he is not a diabetic taking more of simple carbohydrates (sugars) will also help as such a diet gives rest to the liver reducing its burden. Keep a pillow or two underneath his feet to keep them elevated to help drain the fluid with gravity. The abdominal pain is definitely due to the fluid and if it is too distressing then you should get it drained periodically. Another option is placing a peritoneovenous shunt, which will continuously keep draining the fluid from his abdomen into his circulation. But that is an operative procedure and he may not be fit for the surgery. Liver transplant is out of the question in his present state. I am sorry to say, but he does not have very long to live and all efforts should be to make him as comfortable as possible in his last days.