What could be wrong with him?

Does anyone know why a cat would walk this way? He's been like this for 4 days now. The vet couldn't find anything outwardly wrong with him and she checked his hind legs while he was sedated. We're treating him with anti-inflammatories and calici/herpes support medicine because one suspicion was lameness caused by calicivirus. I just want my energetic baby boy back so badly and it hurts to see him like this. :(