My little brothers pet rabbit is dieing, please help?

Awwww...I'm so sorry. If his hind legs are not working and the medicine the vet gave him isn't either, honey he has no chance. It would be humane to put him out of his misery. And I'm guessing that little bunny is not happy being in pain. It's the best thing for him. Death of a pet is awful and you can't replace him, but you can do whats best for him in the long run. Good luck sweetie !