Although I cannot diagnose your dog based on those clinical symptoms, I can suggest some treatment options.
Arthritis can occur in young dogs, and may appear suddenly because we miss the signs of it coming on. Conventional treatments are NSAIDs like aspirin (about 10mg/kg every 24 hours) or some of the newer NSAIDs that you will only be able to get through your vet. I do not recommend Rimadyl.
If this condition does not seem to be getting any better in a reasonable time period (a month or so) you really should try to refer him to your closest veterinary college or referral orthopedic practice. It is possible that he has an injury to his joint that your regular vet was not able to identify.