--that was is going on there is compressive; tarsal tunnel is hard to see on imaging (as is carpal tunnel), unless, as you noted, there is an obvious space occupying mass. And, there are certainly people who experience swelling in that area, especially if they have some circulatory compromise--e.g., they get edema from time to time--which might result in neurologic symptoms.

On the other hand, it's interesting that he's having some reduced response with the knee reflex, as this implies something going on above the foot level. Decreased reflex response at the knee, ankle and elbow does frequently occur with age, though he seems a bit young for that (how MUCH was the response decreased by?)

I'd be curious about the specifics of the nerve conduction/EMG, as well as the MRI of his back (I assume that was lumbar?).

And, of course, it's possible he has the beginnings of something more systemic that is resulting in a "double crush" phenomenon that he is noticing first in the foot/ankle area, which is, of course, one place in the body that is quite prone to compressive effects.

Really not a lot to go on here as to cause, yet--more extensive testing may be needed if symptoms get worse. I assume right now treatment is primarily focused on symptom reduction?