I am a neuroendocrine pancreatic islet cell tumor patient. I have had major surgery to remove my cancer. I am awaiting scan results to see how successful the surgery was. I have not had any chemo yet so I can't personally speak to that issue. My husband has had aggressive chemo and suffered from loss of appetite, lethargy, etc. My suggestion would be to discuss the food issues with a dietician and/or the doctor or physician's assistant. Usually an oncology clinic will have a dietician associated with it.
I remember that my husband depended a great deal on soups when he did not feel like eating. The important thing is to find something of nutritional value that your father can/will eat--even if it doesn't fit the medical idea of the perfect food!