I get quite upset when I see the snake oil salespeople presenting something as a cure that has not withstood any research and review.
Why some people believe that physicians worldwide would know about a cure but "hide" it from people is one of the most ridiculous, delusional and incredible 'ideologies' out there. The outcome to almost all malignant cancers is a terminal one-- unless caught in time and treated, assuming the malignancy responds.
I recall one surgeon discussing this very topic and pointing out that the people on the front lines who do nothing but see cancer patients all day and every work day KNOW about all the alleged treatments and cures. He asked the same quesion, "Do people believe we know there's something that would cure them and that we'd withhold it from them?"
I'm very sorry about the OP's dad. If cancers relapse after the initial response and remission, then the odds of beating it start to fall steadily. Even in cases with a complete pathologic response, there can be a relapse a few years later, though your odds are much, much better if you achieve the complete response vs a partial one. I'd like to believe his oncologist(s) told him about his malignancy and how aggressive it was, survival rates and so on. They honestly do run out of treatment options at some point.