Please don't let one person's opinion color your dad's hope. Humans are very adaptive. I know the surgeon did the surgery and he may have an educated opinion but it's not finite.

He should have a suction machine to get rid of some of the saliva. Sadly it gets worse during the latter days of rads too. I used to spit constantly. Then after that it completely dries up. And may return a few months to years later. The body does heal. Tell him there is hope and a new normal on the other side... Getting through the worst of it of course is the goal.

Hugs.