I have the a310. For poor eyesight, this phone would be wonderful. Its hard to miss the button for voicedial, he can just hit that (Hold down the * key) then say the name rather than type in the numbers. But if he had to type in the numbers, the numbers display VERY largely on the really big screen.
The menus and text are all very easy to read and large because of the large screen. I would recommend this out of any of the phones because of such the large screen. The Motorola T720 and Z800 probably have large screens too, but the color can be hard to read sometimes.
UTStarcom XV-6700 [Verizon Wireless]
Samsung SCH-A970, Samsung SCH-A610, Motorola T730c, Motorola T720c, Samsung SCH-A310, Samsung SPH-A500, Samsung SPH-A460, Nokia 8260