back problems - help!

Hello.  My 8yr old border collie and ?? mix has sufferred from back problems off & on for about a year now.  He has occasional flare-ups when he has had too much activity or somehow strained himself.  Usually a few weeks of Rimadyl & rest takes care of the problem & he is back to his old happy self.  I have also been giving him a glucosamine supplement per vet reccomendation for about one year.  X-Rays have been taken & the doc said that the cartiledge has deteriorated in some places in his spine & the bones in his spine have begun to grow together in order to stabilize themselves.  Recently, he started having pain again, but this time it deosn't seem to be going away.  It has been 1.5 months with little improvement. He is now taking Tramadol 75mg 2xs/day, Dermamaxx 50 mg/day, and Methocarbamol 750 mg 2xs/day.  I have also greatly limited his physical activity w/ the exception of very short walks so he can get out and stretch a little.  What else canI do for him now & what can I do if he doesn't get better?  He is my best friend and I hate seeing him in pain.  Thanks