I'm over 65 and I have enough joint problems to identify with your father-in-law. Without knowing more about his preferences, it's difficult to offer any specific advice. But I'll do so anyway.
Someone recommended something like "just get a GEC or a Case Peanut". Terrible advice for someone with difficulty in fine or gross manipulation. The Peanut, a great small knife, is difficult to open and harder to handle once open for someone with hand problems. Other than one of the Easy Open models (which I've never handled), GECs have heavy springs and I really dislike them for that reason. Great knives otherwise but awful for someone having problems using their hands. Alternatives in the traditional arena would include most of the other Case models which are easier to manipulate. Also look at knives made by Queen--Queen City, Schatt & Morgan, Moore Maker (the more expensive ones only, the low end models are not Queen made), etc. Boker's Solingen Tree Brand knives are good but be careful you don't get a "German" Tree Brand that's really made in China--not bad knives but not traditional Boker Tree Brand knives either.
For one hand openers, it's harder to offer good recommendations. There are so many styles and features and designs. For a good medium sized, well made knife that should please an old guy, I would offer the Spyderco Sage 1, a Benchmade Mini-Gripitilian or Mini-Presidio. I have large hands and the Mini-Grip's handle is too short for me so I prefer the Mini-Presidio--both are good knives. For a smaller knife that's easy to manipulate and handle, I prefer the Spyderco Dragonfly and Cat.
All of these are well under $200 which leaves money for an additional gift or two. Or simply get a selection of traditionals and moderns that cover the range of choices.