The vitamin deficiency certainly could be contributing to his situation, though low Vitamin D is not generally thought of as causing neuropathy by itself, as low levels of B12, B5, B1 and E can.

However, one does wonder WHY his level is so low. Assuming he does not live indoors 24/7--and maybe even if he does--the immediate thought is malabsorption or some imbalance with calcium, which also brings thoughts of gastrointestinal problems . . .and celiac/gluten sensitivity (I don't suppose he's had a celiac panel or mentioned any other non-neuropathy symptoms?).

Of course, celiac/gluten problems have been documented to cause neuropathy through autoimmune cross-reactivity with nerve epitopes independently of nutrient malabsorption effects. (It's certainly possible there are multiple contributory factors. But he really needs to get some more Vitamin D3 in his system.)