I am sorry to hear your cat was hit by a car. Just like with humans, broken bones in our pets can take a while to heal properly.
Assuming the cage rest was recommended to you by a vet I would have a chat with the vet again and discuss options for moving forward. It is important that when the time is right for your cat to be left out of the cage, that exercise is monitored and gradually built up (if that is at all possible!)
Cage rest can be tedious sometimes but unfortunately our pets don’t understand that sometimes this is the best thing for them. Fingers crossed he is moving around normally very soon!