ANSWER:
A believer adheres to the truth regardless of the person who said it!
Your attitude is strange. It is as if you are saying that because he is sinful, you might as well be sinful and you can both go to hell! This is not correct and inappropriate for a Muslim! Two wrongs don’t make a right and he is your father and deserves to be respected and obeyed. You must make Duaa for him and try to advise him in a respectful and diplomatic way while refraining from sins and what angers Allah!