HI there. I'm sorry to hear that your dog seems sore in the back.
Since he's only a year old, I don't think it could be anything like arthritis. The thing that I think of is a probable hip dysplasia or luxating patellas(slipping knee caps). Hip dysplasia is something that some large breed dogs are prone to having. It's where the hip bone of the leg doesn't exactly meet up properly with the socket in the pelvis. Either the ball if the leg is malformed a little bit, or the socket is too shallow, or both, causing discomfort. See more information here about hip dysplasia in dogs:
Another thing it could be is luxating patella's. Hav ing this condition can cause them to "bunny hop" when they try to run. Dogs who have a luxating patella on both hind legs may change their entire posture, by dropping their hindquarters and holding the rear legs farther out from the body as they walk. See more information about it here:
These conditions can both be treated with medications for a while, but it woll not cure them. These are anatomical conditions that must eventually be corrected surgically. Both those sites will tell you what is involved in doing surgery for those conditions.
I hope this helps you out. Let me know if I can help you further.
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