Q. All of a sudden he is not using his hind leg. He limps on it and very frequently stops and starts staring off into space, his eyes moving up slowly.
A. Rabbits are really good at hiding pain. They will sit quietly and sometimes half close their eyes when experiencing pain as they are prey animals, they do not want to show a potential predator any weakness.
Rabbits can have sore hocks if over weight or he may have injured himself. If it gets any worse, he stops using it, or its really bad I would have a vet check him out. Restrict his movement (put him in a smaller cage if he has a large area normally) so he doesn't have to use it too much in the mean time.