I have had polyneuropathy in my arms and legs for 9 years, mostly loss of feeling and tiring out the nerves if I use them too much (woodturning, gardening, close work). Mine is likely associated with celiac (gluten) and/or MGUS (plasma cell disorder) or hypothyroidism. It hasn't progressed I'm not on meds. It would be good to get a full medical eval, to see if your son has other problems. When you have one autoimmune problem, you tend to collect others. ResearchGate has peer reviewed papers on small fiber and other neuropathies.