Oh, no, Stacy! I am so sorry to hear that your little prince broke his leg!
The good news is that everything your son would like to access won't be hindered because of his wheelchair. First, get a letter from your doctor which states any accommodations your son may need. (Will he have a walking cast or crutches while there? If so, he may need attraction access which cuts down on time spent standing.) Take that letter to Guest Relations at any park, which will then give you a Guest Assistance Card. This card will give your family access to the attractions you'd like to try without any concern over standing in line.
If your son is in a wheelchair for the duration of your trip, you'll be able to have him transfer from his wheelchair into most rides with little difficulty. Take a look at the attractions you'd like to enjoy on Disney's website, and click on "Accessibility and Other Information" on each one for more specific information.
Expedition Everest, for example, lists that Guests must transfer from Wheelchair / ECV in order to ride this attraction. (The only attraction that gives me pause is Kali River Rapids, which is only a problem if he has a cast that cannot get wet...you're likely to get SOAKED on that one!)
Regarding entertainment, all of the shows are wheelchair accessible, and you might even find wheelchair-only viewing areas set up in some parks for certain parades / nighttime extravaganzas. Again, check in with Guest Relations when you first arrive in a park for specific tips for your upcoming stay.
If your little man has some healing to do, Stacy, at least it is at the most magical place on Earth! Best wishes for a fantastic visit! Send more questions as you think of them, too. We love to help!