I'm sorry to hear that your father has fallen ill. When it comes to cancer diagnoses, it is always a good idea to get a second opinion. Have you spoken to your senior's health insurance provider? Some insurers actually require a second opinion before covering treatment.
If your father is well enough to go visit another doctor, it would be to his own benefit. If he is not able to travel, ask your senior's current doctor to share medical records, original x-rays, and any and all tests to a doctor of your choosing for their opinion.