There might be a couple of possibilities i would like to suggest that could be part of your differential diagnosis.
-Neuropraxia- If it took place somewhere around december, then, muscle weakness must have already set it. Guess what? by now, he would already have gotten problem with his stability in standing. Do a precise check on his nerve distribution and see whether it correlates with the lateral popliteal nerve distribution. Also palpate the head of fibula cause there's where the nerve coils and makes a superficial appearance. Perform the tinel sign along it's course.
-Musculoskeletal problem- Sometimes, due to nerve impingement/entrapment by soft tissue, the same signs may possibly be seen. So, soft tissue manipulation+myofacial release+ stretching of lateral structures of lateral leg may help. To activate those weak muscles again, make him do blancing exercises on the balance/wobble board or even on a pillow at home.... caution: he may fall.
His back pathology may be totally a separate case altogether or may even be a result of postural compensation with his impaired balance on one lower limb. Tell me if it works yeah!