Hi, his gp can order a few tests, like B12 levels, toxins, diabetes etc the most common causes of neuropathy. but then if nothing comes up he will need a neuro who specializes in neuropathy to do further testin, not all neuro's are neuropathy savy.
The b12 in a multi B complex is not enough if he is down on B12, i would get some B12 Methylcobalamin in supplement form and take 5 mg daily for a few months [ taken on an empty stomache].
Also take the multi B as well [ at a differant time] as b12 needs the other B vitamins to work the best.
best of luck