There are a lot of new drugs to treat arthritis in dogs and they really work. I've used Remedyl and Previcox on my dogs and it works wonders. One of my dogs is a four year old Rottweiler and could barely walk when we started him on Previcox, fish oil and Cosequin. He can actually run now. We also used Remedyl on an old lab and it kept her going for 14 years.
Two of my sisters are professional dog handlers and my daughters run the local animal rescue (ARC of Bay County) and we see a lot of animals with joint issues. Take your dog to a good vet and they might be able to prescribe something that would really improve its quality of life.