My 13 year old lurcher's rear legs suddenly are causing him pain and he cant hardly walk on them.?

My 13 year old lurcher is usually fine on his legs but since Tuesday evening he seems to have a problem with his rear legs and couldnt stand up for long then suddently this morning (Sunday) he started limping and when he laid down he could not get comfortable and was obviously in pain. i took him to an emergency vet who was pretty useless with information other than giving him a pain killer sedative a couple of diazepam (?sp) for this evening. The sedative calmed him down but he has not been asleep and his breathing is laboured. I tried to move him on the sofa and he yelped in pain when I touched his back legs. He eventually got off the sofa and struggled outside to go to the loo. I am taking him to our usual vetst tomorrow and hope they can suggest something like Metacam. I dont want to give up on him but equally I dont want him to be in pain and stressed. Any suggestions would be gratefully received.