@concernedadvocate this is such a tough time. I found the no more treatment visit with my Dad's cancer the hardest of all. He did not argue about it, but followed recommendation to bring in hospice. I think he was tired of side effects and mentally felt there was no way out. He was comfortable and well cared for in his own home until death.
Your Dad's doctor has seen how chemo side effects can be the cause of a hastened death by weakening a frail body further. That is how he makes his recommendations. I would hope there is a doctor out there who would fulfill your father's wishes of more treatment, but if not, reconsider that their viewpoint may be correct for offering the longest life to enjoy, rather than suffer.
I have know people who said that their time spent on hospice was the absolute best of all medical care. If his disease is not curable at this point, then perhaps hospice or palliative care will give him comfort and acceptance, rather than the belief that he is giving up. Talking with a hospice social worker would be helpful. A second opinion from a referral type cancer center, even if virtual visit, would be helpful.
A Dr. Ed Creagan from Mayo who specialized in palliative care has a couple of good books to read. I love his book "How Not to be My Patient" with many suggestions for doing all that you can for your own health preservation. Explore Immune therapies like Beta Glucan for strengthening your own ability to fight cancer. Medicinal mushrooms are another complimentary therapy that can be used on your own. Another good book: "The Rebel's Apothecary" By J. Sansouci. I have used an immune therapy for 12 years after my initial cancer surgery and chemo/radiation. When traditional treatment stops or takes a break, doing these other therapies gives you the feeling that you are still fighting cancer, not just waiting for it to come back. Using something that doesn't hurt you with side effects, and might help you, can be a positive thing. My Mayo oncologist feels that these other things I have done have caused my cancer to behave abnormally with less aggressive behavior, and allowed me to respond better to my last immunotherapy drug. I feel that everything I am doing is helping to prolong my life, and attitude is so important. Perhaps this could help your Dad also.