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Hello. Thanks for the question.
I would at the very least go consult with an orthopedic specialist. It sounds like he probably has an ACL tear in the leg. That is the most common rear limb injury that leads to a non-weight bearing lameness. It's not always easy to appreciate on orthopedic exam. In people, it's an MRI diagnosis. In dogs, we do it by feel. Ideally, a sedated exam will allow us to feel the laxity in the joint and confirm the suspicion.
If this is an ACL tear, these are treated surgically. Without surgery, they build up significant arthritis and the instability leads to a chronic lameness. There are some braces that can help some, but ultimately the prognosis is guarded to poor without surgery. Here is a nice link on the condition in dogs:
I hope this helps, let me know if you have any other questions.
Sorry for the delay. I didn't see this reply until now.
I would definitely get an ortho consult. They may be able to give you a diagnosis without a sedated exam. If not, then some mild sedation is perfectly safe. From there, you can go over surgical or non-surgical options to improve his quality of life going forward.