Maybe this is a neurological problem? If so, it wouldn't show in blood analysis or an X ray.
The good thing is there appears to be no gross damage or that would have shown on the X ray, and it's good news that his blood results are okay too.
Also if he is acting normally and appears to be in no pain, can walk, balance correctly etc, then that is good.
I am trying to think what might help diagnose the problem....possibly an MRI or brain CT? But if all else seems well, it may not be great to subject him to that, plus anesthetic, and to just leave him in peace.
And I guess he's had no local injury to his leg or foot which may have caused local nerve trauma? (but that would usually present with pain of some kind.)
I hope he will be well anyway. I'm sorry I don't have a better answer.