Reply #4: Write. It. Down.

[gratuitous]

Put it in writing, have your dad sign it or initial it. Make sure the wording is exactly to his liking. Put a date on it, too. Then bring it up every six weeks or couple of months or so. For example, "Europe will be Muslim soon." Ask him how that's coming. Has an imam replaced the president of France? Or the taxes thing. That will be an absolute rib-tickler come college bowl season.

Handled with humor and a certain resoluteness, you might - just might - get him off some of the loonier talking points. If, by some miracle, his brain clicks on again, his Fox News-inspired insanity might just vanish like frost in the spring.