Hey @woozy there are lots of Chromeboxs available that would be perfect for him, I'm attaching a link for one below. I moved my late eighty-year-old Dad over to a Chromebox years ago when he was alive and it was the best move we made. He was able to operate it easily and the maintenance required by me went to just about zero. You need very basic specs for him, likely an i3 processor and 8 GB of RAM. He can use his existing monitor, keyboard, mouse and speakers. No need for antivirus and the updates will happen in the background and appear when he boots up.
As @kemtue suggested, downloading ChromeOS Flex will give you the OS you can install on a PC for testing. The advantage of an actual Chrome device is it will also run a lot of Android applications which ChromeOS Flex won't