My daughter broker her leg when she was 3 1/2. Her cast was mid-thigh to foot (just the toes out), and she had it for 12 weeks. The first full week is absolutely the hardest, and it will definitely get easier.
On of the things that helped her was that it was during the school year and she was in a mother's day out program, which encouraged us to get back "out there." And hanging with her friends forced her to move around.
Once you think your son is ready (not in real pain), I would invite a friend over to play. Have some toys that they can play with on the ground together (legos, trains, cars, etc.).
I would also suggest some fun outings where he needs to use the wheelchair or you can't go. Some suggestions include:
Founders Plaza (DFW Airport Observation area)
Explore Gaylord Texan
Fish Feedings at Bass Pro
http://familyeguide.com/Indoor-Play-Areas.html#BassPro
Library Storytimes (they have some great performers during the summer)
http://familyeguide.com/Summer-Reading-Programs-2010.html
Summer Stampede Racing at TX Motor Speedway (only $5)
http://familyeguide.com/Summer-Stampede-Racing.html
World Series Youth Baseball (all next week in LV/FM)
http://familyeguide.com/World-Series-Flower-Mound.html
Texas Firefighter Olympics (all next week in Lewisville)
http://familyeguide.com/Firefighter-Olympics.html
Meet and Greet Soldiers at DFW Airport
http://familyeguide.com/Support-Our-Troops.html
Tour a local business (something that has lots of room for his wheelchair, like Market Street or Costco) http://familyeguide.com/Tour-Guide.html
A couple more lists that may be helpful for activities:
http://familyeguide.com/Free-In-Flower-Mound.html
http://familyeguide.com/Boredom-Busters.html
One more thing... the week that we got her cast removed was also an issue. Because she had spent 3 months with it, there was a strange attachment, and some fear. She wouldn't even attempt to walk on her leg for a day or two, and would cry that she wanted her cast back. I don't know if that happens with a lot of young kids, but just be prepared.
Good luck! It will definitely get easier.