Getting a feeding tube (and it may not be a PEG, but a G tube to the stomach - the only difference is how they put it in) can definitely improve your father in law's nutrition. Unless something else is happening medically to your FIL, he can live on his feeding tube until he dies of other causes.
I have been continuously on a feeding tube for over two full years and expect to do so for 15 to 20 years more.
Bottom line: just getting a feeding tube does NOT mean the patient only has a number of months left to live.
I have to admit that your worry/concern was exactly how I felt about a feeding tube. It was only for very very sick people who were going to die within months. That was the cinema image I had and one reason (out of many) why I was so stubborn in refusing to get a PEG my first year of TX.

Don't be confused by any of the posts here about PEGs, those discussions and opinions are almost entirely in the context of patients who get them during TX and then have them out later. It's the difference between wearing the "boot" and using crutches for a broken foot and being in wheelchair for life.

Don't underestimate the hard psychological hit your FIL may take from having to get a feeding tube that may become permanent. Encourage him or his wife to log onto OCF for questions. Get prescriptions for liquid nutrients so insurance can pay for it rather than buying at the supermarket. Please tell him I turn 64 this year and I am doing just fine with my permanent feeding tube. He can do it too. Congratulations on your baby to be
Charm