I would go to:
http://pages.prodigy.net/stevesoares/index.htm -- Steve Soares' page. It lists a lot of the performances throughout the parks. Sometimes, the times he lists aren't always accurate when you get to the parks I've found (but he's usually pretty accurate). Grab a times guide for whatever park you attend in addition to the listings here. There are plenty of things to see even if your FIL never gets on a single ride.
I would also read over Deb Will's page:
http://allearsnet.com/pl/special.htm
for visitors with special needs.
Don't forget the many theatre presentations: American Adventure, the four 3-D movies, Hall of Presidents, Impressions De' France, etc.
AK - this is the park I'm personally least knowledgeable about, but, by the time you go, the Finding Nemo musical will have opened. I personally am afraid of birds so haven't seen Flights of Wonder, but have heard it's great. There's also the Lion King and Pocahontas shows here AND a parade. There are lots of paths in the Oasis and there are the two "walking" paths that he should be able to maneuver through (I've seen others on these
scooter things there)...the ones with the gorillas and the tigers. While the rest of you ride Kali River Rapids, he can certainly be entertained watching the monkeys at the temple (I don't ride the ride 'cause I don't want to get wet, but I love the monkeys). Also, try to spot DeVine somewhere in the park!
Epcot - My gosh, there are LOTS of things to do/see! Turtle Talk with Crush is adorable, I did see that Finding Nemo ride has easy access for people in chairs. There is assistance available to get onto Spaceship Earth. ALL the countries have some kind of entertainment (perhaps the only one that he may have trouble with would be Mexico, since he would have to climb stairs to enter). I love the acrobats in China, the juggler in Italy, the drummers in Japan, Off Kilter in Canada, the World Showcase Players in both UK and Italy, the balancing act in France, the candy maker in Japan, British Invasion in UK, etc. He may also enjoy the character connection. Don't forget an after dark fountain show below Spaceship Earth. I would also recommend a trip through the "House of the Future" in Innoventions if he likes new gadgets (it's where we FIRST saw a TIVO back in 1998). Illuminations!!
During Christmas, there are also storytellers in each country (not just for kids, 'cause my partner and I don't have kids and we loved it), the lighting of the tree/grand arch is awesome. The arches are synchronized with music and are incredible to watch once it's gotten dark.
Of course, snacking around the world is also pretty fun!
While at Epcot, he may want to also take a ride over to the Boardwalk...four restaurants, a great bakery and some shops. Also usually some carnie-style performers after dark (jugglers, etc.).
MK - a great deal of the rides have handicapped access. There's also the parade/fireworks/castle shows. Also the Dapper Dans on Main Street, characters throughout the park including the new "be a Pirate" Jack Sparrow show and a new country music show featuring Woody/Jessie. Don't forget getting a Dole Whip at Aloha Isle when in Adventureland!
MGM - Indy Jones' show, Walt Disney: One Man's Dream, MuppetVision, Little Mermaid, Motor Lights Action, the general atmosphere, the Osbourne X-Mas lights on New York backlot, Fantasmic, the streetmosphere performers throughout Sunset Boulevard. I would think that the Great Movie Ride would have handicapped access.
During X-Mas, I would suggest at least going to Grand Floridian if not the rest of the resorts (which are all decorated lovely) to see the huge gingerbread house and have some cider/cookies.
I've not seen Cirque De Soliel, but I'm sure that there must be handicapped access at that theatre as well.
Most of all, enjoy...relax, people watch (one of my favorite activities when at WDW), appreciate your time together!