A. This sounds like either a serious traumatic injury (dislocated hip joint) or a neurological abnormality. Cats are known for jumping off things and landing incorrectly causing hip dislocation. I would have him checked by a veterinarian and if possible x-rays done on the leg to look for signs of trauma. Based on what they find a good treatment for this can be implemented, potentially surgery.
However if this is by chance a neurological abnormality, they are a little more involved as far as additional diagnostics and treatment.
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