Welcome and you came to the right place. We will try to help you out.
You can still go and have fun. It just will be a little different than what you had in mind.
1) Even if he is using crutches, don't plan on him walking the whole day. It takes more energy to walk with crutches. Plan on renting a wheelchair; keep in mind that the wheelchairs rented by WDW are adult size. He may also fit in a stroller. If you use a stroller, you can get a Guest Assistance Card from Guest Services that allows the stroller to be used the same as a wheelchair.
2) If you are staying at a WDW resort, you can get a wheelchair from them (limited number, first come first served) that you can use for your entire stay. This would be a big help in getting around the resort.
3) Here's some information about casts that I copied from the FAQs at official Disney site:
Q. Which attractions would I be able to experience while in a full leg cast?
A. The following attractions have additional space on aisle ways or next to the seating areas and should be able to accommodate a full leg cast. Please speak with a Cast Member at each of these locations for additional information.
Magic Kingdom
Walt Disney World Railroad
The Jungle Cruise
Liberty Belle Riverboat
"it's a small world"
Cinderella's Golden Carrousel
The ExtraTERRORestrial Alien Encounter
The Timekeeper
Walt Disney's Carousel of Progress
"The Enchanted Tiki Room Under New Management"
Country Bear Jamboree
The Diamond Horseshoe Revue
The Hall of Presidents
Legend of the Lion King
Galaxy Palace Theater
Epcot
Body Wars (Wonders of Life)
Ellen's Energy Adventure (Universe of Energy)
Cranium Command (Wonders of Life)
The Making of Me (Wonders of Life)
El Rio del Tiempo (Mexico)
Wonders of China
The American Adventure
America Gardens Theater
Impressions de France
O Canada!
Honey I Shrunk the Audience (Imagination!)
The Living Seas Living with the Land (The Land)
Food Rocks (The Land)
Disney-MGM Studios
Star Tours
Twilight Zone Tower of Terror
The Hunchback of Notre Dame -- A Musical Adventure
Muppet*Vision 4D
Indiana Jones Epic Stunt Spectacular
Sounds Dangerous
Disney's Doug Live
Backlot Tour
Backstage Pass Walking Tour
Voyage of the Little Mermaid
Bear in the Big Blue House, Live on Stage
The Magic of Disney Animation
The Great Movie Ride
Beauty and the Beast, Live on Stage
Fantasmic!
Disney's Animal Kingdom Theme Park
Kilimanjaro Safaris
Pangani Forest Exploration Trail
Wildlife Express to Conservation Station
Conservation Station
It's Tough to be a Bug
Safari Village Trails
Festival of the Lion King
Pocahontas and Her Forest Friends
Maharajah Jungle Trek
Flights of Wonder
Tarzan Rocks!
