Bigger dogs are often at risk of getting arthritis in their joints, and this sounds a lot like arthritis. I would take your dog for a check-up at the vet, they may be able to give to some medication for your dog to reduce his pain.

I did a bit of research to double check information I've shared with past clients. It does sound like he may be starting to experience some joint challenges, likely arthritis. ALTHOUGH..I would urge you to have him Vet Checked to be sure before doing anything. Also become more informed as to what is really available and what the pro's and con's, such as SIDE EFFECTS are BEFORE you choose a course of action if it IS arthritis. The first is a Google general web site that offers many links to learning more about holistic treatments (homeopathy...which I have used and DOES OFTEN SOLVE THE PROBLEM with little or NO side effects) and other alternative treatments. http://www.google.com/search?q=homeopathic+treatment+for+arthritis+in+dogs&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a http://www.nativeremedies.com/petalive/ http://www.pets4homeopathy.com/remedies.html The second Google addresses the challenge from a more Western Medicine (typical) treatment perspective that you are likely to encounter, unless you have a Holistic Vet. Included are some links as to the possible side effects to be aware of when using this course of treatment. http://www.google.com/search?hl=en&client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial&hs=yyv&q=+treatment+for+arthritis+in+dogs&btnG=Search http://www.srdogs.com/Pages/rimadylfr.html http://www.petstyle.com/dog/health_well_article.aspx?id=1962&section=Health%20amp%20Safety For mild cases, the Vet may also suggest using BUFFERED aspirin...ONLY buffered should be used, to avoid stomach upset and the chance of the dog developing ulcers, just as some humans would. Your Vet may also want to do some blood work, I encourage you to do it. You want to be sure it is NOT cancer in the one leg, as well as get a base line accounting of his body's general function and condition (Liver, Kidneys, Heart) as their condition will also have a baring on the choices your Vet can offer for treatment of any condition this turns out to be. Even if your Vet knows nothing about the safer Holistic treatments, or is AGAINST them, if your dog already is starting to show signs of problems with the liver or kidneys, it is unlikely that the Vet would suggest some typical treatments...because they are known to CAUSE kidney and liver dysfunction! I've been here too with older dogs, so my heart goes out to you...but he sounds like he has strong opportunity to continue living a happy, good quality of life for some time yet...with a bit of support!