No conversation technique will enable you to always get what you want. I mean, if such a technique existed the other part could also use it, right? The guide is not about how to win a conversation, just about how to have it.

Your state that "the power went to his head" and he is "irrational". That is pretty judgemental! But looking at your story from the outside, he is the son with the dying father and you are an in-law trying to wrestle control of the situation and get access to medical information and perhaps influence critical decisions.

No amount of conversation skill is going to make you closer related than the son.