ChromeOS. Easiest OS to use. It's just a browser. Does he have a real need for native applications? Can he get everything he needs accomplished inside a browser (Google Docs, email, etc)? How much of what he's doing now is in the browser anyways?

I switched my grandfather over to it this year and he loves it. No more notifications / nags for updates, adware, and such. All extraneous pain points from windows are stripped away.

Just works.