How long ago did he get this injury? If it was recently then he may just need time to heal. Does he seem to limp or favor a leg when he walks? He possibly could have just injured a muscle or ligament. But, like others said it could be his knees or a back injury. I would have thought the vet would have rules those out. My advise is that if this has been going on more than a few days and he is still in pain then I would find another vet. I also recommend you keep him away from the other dogs and put him some place where he can get rest and not get injured again. Is he eating and drinking okay? How big is he? Please keep us posted.