I would look for a S&W snubby for him in .38. The older model 60's and 640's are .38 special versions and have better triggers than the current, and the all steel frames will make recoil more manageable. If you go with the airweight revolvers he can always stick to standard .38's if the recoil of the +p's is too much. Also depending on his hand strength a good trigger job and a 14lb rebound spring will help alot with the trigger pull. I do not recommend changing out the mainspring since it could effect reliability and cause light primer strikes.
Now if this is strictly a home defense gun I would go with a model 10 as others suggested. The trigger pull is buttery smooth and even an old timer should have no problem with it. I would rule out semi-autos because I do not know many 80+ year olds who can manipulate a slide very well.
Here is my 640 I picked up for $300 used. Put a 14lb rebound spring from the factory 18lb, and stoned the trigger and polished the internals. I have yet to feel a better trigger on a J frame.