I'm a little late to the discussion, but I have two cents to offer:
I like some of the suggestions so far. I am quite partial to a well seasoned Seagull. No, not for dinner.
Forgive me for speculating, but I am guessing that you want a top tier guitar that will provide him with years of pleasure. Something that will be a part of so many fond memories that it eventually becomes a family heirloom, right? Seriously, a vintage Guild fits the bill. Get him a Guild dreadnought. An old D25 is arguably the best six string guitar you will find for under a grand. It is a buyers market right now and they often pop up in your price range.
For instance, here's a nice D17 with a pickup for $450
http://greenville.craigslist.org/msg/1629194332.html
The GAD series was mentioned already, but I think you should hold out for a nice used Guild D series made by craftsmen in the good ol' USA. These instruments were designed and built for players. They are very comfortable, well built and sound fantastic. As an added bonus, your Dad will probably get a kick out of joining the folks over at
Let's Talk Guild. It is a very -- and I mean this with great affection -- "Geezer Friendly" community full of wizened music veterans and retirees enjoying their fine guitars.
(In the interest of full disclosure, I owned a bunch of different guitars for the better part of two decades before I found my Guilds. My left hand often cursed my name. Now, I think back and wonder what took me so doggone long.)