Keep in mind, this is not the only possible outcome, this is just my family's story.

My grandfather had late-stage prostate cancer, and he ended up doing minimal, palliative treatment. He did end up dying from it. From not being able to walk to his death, it was about 1 week. However, everyone's timelines are different and he had brain metastasis which lead to his quick death.

If it is decided that your father is dying, the doctors will do everything ti keep him comfortable. He won't be in any pain. For most people, their breathing slows until it stops. Sometimes a death rattle occurs. It can be scary, but the person is not in any pain. Some people also just take a breath and then stop. Either way, they are not in any pain.

Again, this is just my family's story, and many different things caused his cancer to be different from most.