Maybe his hip or ankle popped out of his socket/got dislocated??? was he recently picked up & wriggled like crazy or see him try n squeeze thru a really small space??? That's a tough one. If you have the means, I'd say go to the vet & find out what's really going on. As I'm sure you know, their bones are pretty fragile and just them struggling in someone's hands can cause things to pop out or break 
If you can't bring him to the vet, take another rabbit and feel around on the back right leg then feel his & see if there are any differences- & if nothing feels out of the ordinary(no bones out of place puncture wounds, cuts) for sure try wrapping his leg again, wrap it & then wrap around his midsection to keep the leg in the most normal alignment possible - if you feel something wrong def vet no matter what.
I hope he gets better soon!!!!

If you can't bring him to the vet, take another rabbit and feel around on the back right leg then feel his & see if there are any differences- & if nothing feels out of the ordinary(no bones out of place puncture wounds, cuts) for sure try wrapping his leg again, wrap it & then wrap around his midsection to keep the leg in the most normal alignment possible - if you feel something wrong def vet no matter what.
I hope he gets better soon!!!!

