Last year, my father stopped his blood pressure meds and was very surprised when I could guess he'd stopped just by looking at him, then proceeded to explain how the drugs acted, and got even more surprised when he got much better 15 min after taking them again. Since then, he totally forgot about this event and is in complete denial. You can't change beliefs by explanation. People have to get to their own conclusions, otherwise you're just fighting windmills all the time.
>he totally forgot about this event and is in complete denial.
I can never understand this, especially with well educated people like your father or mine. I can't get mine to agree to go see a doctor (other than me - he doesn't believe me when I tell him something about medicine) for anything, even when there are obvious problems with his health.