Interesting point. So far it's not a problem - he's 70 next year and physically stronger than me (that's the IT lifestyle for you!) and doesn't have trouble with fine motor action. My grandfather did computer courses at the local library in his late 80s, maybe 90, and he did suffer from arthritis but still managed.
At this point I'm more worried about the software limitations. He may decide he wants to write a letter, and a tablet would be a pain for that (and printing - again, any cheap printer will do with a PC but there's more to consider for tablet compatibility). I just don't want to hit a point where he's frustrated at having spent his money on a tablet that can't do what a laptop would do for around the same sort of money.