How can one deal with a 70-year-old father who does not pray, and argues with the intercession of the Prophet, peace and blessings be upon him, and is very angry and unapproachable for discussion? And would he be considered a disbeliever if he dies in this state?

Your father has committed a grave major sin by abandoning prayer, and scholars differ on whether this act expels him from the fold of Islam. You must advise him with gentleness and kindness, explaining the seriousness of his action and that it is worse than adultery and theft. Furthermore, clarify that Allah's mercy and the Prophet's intercession are attained through faith and righteous deeds, not by persisting in disobedience. If he is guided, then praise be to Allah; otherwise, you have fulfilled your duty. Strive to supplicate for his guidance.