I remember there were a thread about tips for visually impaired. I searched for 'visually impaired' in the forums and couldn't find what I remembered, but I did find these three threads that might be of interest:
https://www.mobileread.com/forums/showthread.php?t=42923
https://www.mobileread.com/forums/showthread.php?t=39757
https://www.mobileread.com/forums/showthread.php?t=8742
The largest size on Kindle 20 pt - I don't know if that is big enough. You also have to consider which books he is going to read - what format(s) and are you prepared to convert (and perhaps de-DRM).
For a Sony, if you can convert, you can use calibre and set the base font size to a very large one - that way you can get around the size issue.
Then there's the Hanlin V3 clones, among them the Australian ECO Reader. You could ask here about specific issues:
https://www.mobileread.com/forums/forumdisplay.php?f=101