My experience (a few deaths recently) has led me to believe that the body knows when the end is nearing. I hope this is not a bad thing to say to you but maybe at 85 his time is near. It seems that people often start to withdraw in many ways when the end is nearing, and that is nature's way of allowing us to emotionally leave this earth.
It's sad, but if he doesn't have it in him, no amount of doctors are probably going to make a difference, and people do have a right to live their later years the way they want.
Of course, please encourage him and your grandma to consult a doctor if you wish, just don't expect it to necessarily make a difference.
You must care about him a lot.