depending on how much responsibility you’re willing to take, and how far you’re willing to go. (assuming you are not depending on your dad) show him some tough love, and if he doesn’t change, become tougher. take away his food, if you supply money or do the shopping, don’t buy any junk food. argue, yell, tell him what you know is right. everyone has a breaking point, even your father.
however, if you don’t feel like taking this approach, as it can be extremely mentally taxing and permanently damage your relationship with your dad, then there isn’t much you can do. his life is in his own hands, ultimately, he will do what he wants.