My DD was in a cast similar to that, with casts all the way down both legs including her feet, when she was 6 (she had surgery on her hips and ankles). She also a spreader bar to hold her legs apart.
We found a bean bag chair was very helpful; not only did it allow a different position, but the beans help to releave pressure compared to sitting on something more solid.
School has to set up home bound teaching if he can't get to school. So, it's not your responsibility by yourself to come up with a program for him. I'd suggest making a schedule for each day, with time for each school subject, reading to him, him reading to himself, playing games. With a schedule, he won't get as bored.
In a hip spica cast, he probably won't fit in a wheelchair. We were able to rig up DD's wheelchair so she could still use it, but it involved using some pieces of plywood to support her legs. My DD wouldn't have been able to use it, but I've also seen people do something similar with a
scooter board (those things like mechanics use to "scoot" under cars.
Depending on his size and the cast position, he may fit in a reclining wheelchair.
In the hospital, they had some wagons with sides that could be removed and you might be able to find something similar. An outdoor reclining lounge chair might also work for a change of position (and it's light so you can move him to different parts of the house).
Aside from that - just sending some pixie dust for a quick recovery.