Poor boy. There are few things I recommend-your baby sholud have x rays done to make sure this is actually arthritis- not to scare you but dogs can get bone cancer as well! There is a medication called METACAM that is like a miracle drug for arthritic dogs! It is an anti inflammatory but works on a different pathway than the Rimadayl. He would have to be off the Rimadyl to use the Metacam. The Metacam can also be combined with Tramadol, a pain medication. This combiination works wonders. THere is also a condition called Degenerative Myelopathy that be a contrubtuing factor which affects the nerves in the hindend. There is minimal treatment for this. There are special diets and supplements that may help and are worth na try!
If you have further questions just let me know.