Changing UI's have caused him to eventually give up using the computer...
I'm not quite as old as your father, but I still prefer the type of UI that I used back in the days of Win95 because it does things the way I expect, and doesn't have an ever-growing set of bells, whistles and gongs getting in my way. That's why my computers all use Xfce, because it's trivial for me to set up the way I like it. And, for somebody like your father, who isn't interested in knowing what's going on "under the hood," switching to Linux isn't hard at all, especially if you pick a distro such as Xubuntu, which is designed with new users in mind.