Sorry to hear that your Dad is having these other issues that are preventing his treatment for the cancer, itself. I'd guess it is difficult for anybody - even doctors - to predict just how long somebody might live with multiple things happening in the body.
You still might be able to have Palliative Care, even if not doing a cancer treatment. Ask about
it at whatever hospital you would take him to in the event of a medical downturn. Maybe also talk with them about
the options for Hospice Care, too. Some of the specialist in that area can help make him more comfortable, and also provide help and respite for struggling caregiver(s).
I would also make sure that everybody is "straight" on what you Dad might want done/not done in the event of his serious medical decline, or approaching death. If applicable, refresh any Power of Attorney for Health Care, or Advanced Directive for Health Care documents. Having to go to the hospital and "muddle through" these issues totally unprepared can be really difficult.
Last year I knew a man who eventually died of complications from his advanced prostate cancer. In the last months the family had to deal with mobility issues because of some stairs in and around the home. They even went so far as to find and install a small lift that would get a wheelchair up onto the porch. He had a BRCA mutation and had been treated with the PARP inhibitor Lynparza and also a tough Carboplatin chemo. He too, had to stop as adverse events and side effects began to pile up. He did not die of the prostate cancer, itself, but his body formed uncontrollable effusions in and around his heart and lungs, which gradually took their toll on all organs and his heart. The most rapid decline happened in a period of a month or less, particularly after they discontinued some interventional draining of some of the accumulating fluids. Each experience is individual and unique.
Your family may have already seen this video before.passing-on.org/
These are hard times. But they can also contain indescribable moments of love and caring, and sometimes even some laughter and hugs.
Take care. Keep posting. Vent if you need to.
Wishing you the Best,
Dx Nov 2013 Metastatic Prostate Cancer at Age 65
Numerous Bone Mets and Lymph Nodes, PSA 5,006
ADT Lupron + Zometa, PSA Nadir 1.0
Resistance after two years.
Short rechallenge Casodex.
Oct 2016 Provenge
Dec 2016 Start Xtandi, PSA 95
Sep 2017 Xtandi PSA Nadir 1.2
Apr 2018 PSA 2.0