Major surgery is a terrible thing to contemplate. It is scary and hard to get your had around how such a thing progresses. Everyone deals with such things differently and doctors because of how modern society works must discuss the worst case scenarios that makes things even more terrifying. My wife has had a two heart valve replacements and had a third one (mechanical valve failed) done. (two valves two surgery) It is beyond description. There is nothing I could say to describe how terrifying this is. I completely understand why your father in law (ok not quite but...) is afraid. Anyone would be, the question is the result of not doing the operation is also quite scary. My uncle about the same age had a major stroke 7 years ago. It has left him with little hand control (he cannot write) no speech, little ability to walk, and he needs to eat pureed food and have 'thickened' drinks. My wife also has had a major stroke and have reduced mobility and and mother control.
Speaking for myself only!
The most terrifying thing I can think of is being in my uncles shoes, not really mobile, not able to communicate, not able to eat solid food, and being fully aware of everything around him. I cannot imagine how mortifying it is to be out with your nephew, not have control of your bowels and have to be cleaned up by him. I have looked into his eyes and I absolutely know the depths of that soul destroying moment.
It is a terrible moment you are in. There is no good answer or better option, nothing to minimize the impact. It is a an life altering moment. At the end of the day it is his decision. From the depths of my soul I wish this was not happening to your family. My thoughts are with you, I hope for things to brighten.
Alex.