Spinal tumor is what comes to mind also, unless there's some abcess there on his back. I recall Shadrach had a chicken that had a bad injury like a falling tool hit it's back, and he rehabbed it, but this sounds progressive like a tumor.And you can’t feel any hard mass that could be an abscess? Probably not likely as it is both legs.
Bob is thinking sepsis - ie systemic infection in the bloodstream, but he would be very unwell with that.
And you have tried the magical vitamin B2?
From your description it sounds to me like a spinal tumor. Poor chap.