Has he had the skin biopsy? Had it done twice progressively worse second time.
There are several different causes for neuropathy. Chemo,
Medication induced like statins, idiopathic and autoimmune.
Go to the list medications. Gives other information https://www.foundationforpn.org/wp-content/uploads/2016/10/Medications-that-Can-Cause-Peripheral-Neuropathy.pdf