Grandpa should probably use a chromebook. It's cheap, it's not as targeted as windows systems, and doesn't need AV. On the downside he'll be locked into the Google ecosystem, but for his needs that might not be so much of a problem.
If it becomes a problem, GalliumOS is actually good enough in most cases to use as a daily driver on a Chromebook.
Grandpa will be better off with an iPad since iPad has way more apps than Chromebook and is more intuitive and user friendly than Android or Chromebook. iPad can can work offline, more portable and convenient to read (pinch and zoom etc) and to write, type with a keyboard that you can connect or touch type on screen. Not to mention Apple will protect his privacy more than Google will.
I see where you're coming from and my (then 76 year old) Gran loved using my iPad mini, but she found the screen too small and a full sized iPad too heavy to hold. She did like her iPhone, but would still routinely send random messages to the wrong person.
She would've also found the jump from iOS 10 to iOS 11 confusing, as she did from XP to Windows 8.1.
I'm only posting this because I went down the exact route you mentioned for exactly the same reasons and it backfired spectacularly, and expensively.
If it becomes a problem, GalliumOS is actually good enough in most cases to use as a daily driver on a Chromebook.