There are many different diseases which can cause what's known as "shifting leg lameness". Several of them are tick-borne diseases like lyme disease, ehrlichiosis, and anaplasmosis. Tick borne diseases are generally responsive to treatment with doxycycline. Another possibility is immune mediated arthritis, infectious arthritis, or vaccine induced polyarthritis. Young growing dogs are also susceptible to a couple of conditions known as panosteitis and hypertrophic osteodystrophy. I would suggest doing tick titers and xrays to start. Your vet may also want to "tap" his joints to analyze his joints to analyze his joint fluid. Possible medications would include doxycycline, prednisone, and Non-Steroidal Anti-inflammatory Drugs (NSAIDs) such as Rimadyl. Good luck with your little man!