I gave my Dad (86) my old iPad1 and he reads very happily on that - he prefers it to his Kindle. At a certain point, I think it becomes less about the sharpness of the screen, and more about the size of the words

EPUB may be an 'open' standard, but the DRM isn't (e.g. ePubs from Waterstones use Adobe Digital Editions DRM, ePubs from iBooks use Apple's FairPlay DRM etc..) Yes you can strip the DRM, but does your Dad (or you) really want to do that with every book? Amazon's Kindle format is based on the old MobiPocket, btw - Mobi was far and away the most popular DRM'd eBook format around when Amazon was introducing the Kindle, and Amazon bought them out around 6 or 7 years ago.
+1 for the most flexible option being a tablet of some kind, as there are almost certainly apps available for whichever flavour of format and DRM you can think of - my wife used to use a Sony Reader, and can still access her library of ADE ePubs using the BlueFire App on her iPad, for example. But eink readers, albeit less flexible all round, are the cheaper option.
HTH Pete