Maybe by being after the father-in-law to improve his eating, he might eventually listen and do it, thereby accomplishing better health with which to enjoy life to the very end
The bigger question is just how do you go about convincing people to change? Everyone has a different idea on how present our way of eating to people, and some have more success than others.
I have found that a confrontational in-your-face approach just doesn't work with most people. It works for Dr. Esselstyn, who works with people who have very limited options and are already "up against a wall". It doesn't work for most others, though. Can it work for someone who is 93 and, according to the original poster, has a "nanaannaa attitude" about it? Obviously not since the original poster was told to "get off his back" about it. That's why my advice was:
Lighten up. Offer kind encouragement where you can but don't harp, it's not likely to work and will only make everyone else ignore you too.
I wouldn't abandon him, but I'd back off and try a much softer approach.
People don't like a softer approach because they don't see it as being effective. Truth is... it's not very effective. But it's far more effective than being pushy about it. So why are people pushy about it if they know it's not effective? I think because in their frustration they feel that at least they're trying. Sad consolation, if you ask me, for being even less effective.
How successful am I with my "kinder, gentler" approach? I can't know for sure. I've helped people change that I didn't even know about. Just the other day a lady approached me and started talking about it. She and her husband are now McDougalling and she's ordered his new book. I had never spoken directly to her about it! She overheard me talking to other people a couple times. The McDougal way of life is natural, satisfying, and rewarding. People will naturally be drawn to it if we present it in a way that isn't confrontational, demanding, or demeaning. For some of us that means having to put our argumentative personality aside and letting the rewards speak for themselves.