I'm 62 and recently spent a good deal of time in the hospital (two major surgeries 3 months apart) so I can empathize a bit.
1). He will probably appreciate a larger iPad rather than the mini. I say this because unless all he does is read books the larger size will be easier on his eyes. If he plans to read a lot get the mini because its easier to hold for long periods.
2). Get one with some storage. I put a couple of seasons of my fav TV shows and a couple of Monty Python movies on mine. Good light hearted entertainment. Hospital wifi was just no good for streaming. My 64G is almost full but worth it.
3). Speaking of easy to hold - get a good case with a stand or at least a stand. An iPad gets pretty heavy even to just balance if using it for long periods and he will likely feel pretty week. I have a STM Grip on mine because the case is really light but has a built-in stand with two angles. Works well on hospital tables. This is important. The iPad will do him no good if he can't prop it up.
4). Doesn't much matter if you get a 2 or a 3, if your budget is limited go for a 2 with more storage. Depending on how much time he has to spend away from home you might consider cell service. As I said, I was in the hospital and the wifi there wasn't much good for much more than a bit of slow surfing. I used my cellular service to Skype my husband and grandkids because my first surgery was done about 2000 miles from home. (I was on vacation with my sister for that one).
Hope everything goes well for him. Load him up with books, games, movies etc. giving his mind something else to think about will be well worth it.
Best wishes to your dad and your family.