My 4 year old son broke his femur last year. He wasn't in a cast but was in a wheelchair for 8 weeks (he had TENS placed in his bone).

It was really hard to keep him occupied but he found use in a Nintendo DS, maybe your friends little boy could borrow a Leapfrog hand held game or similar. Colouring and things were difficult at times as he would try to balance a tray on his wheelchair but it was awkward. The other thing was watching DVD's or ABC kids. I found that through his injury he did get tired a lot easier than before.