have you been thoroughly tested already? spinal tap? blood tests? skin punch biopsy? emg/ncs? a nerve biopsy is a last resort. very few people who have posted here or that i have talked to in real life have had a nerve biopsy and i can only remember one who had one who said it aided in diagnosis. the fact that the doctor wont tell you what he suspects or is looking for in the nerve biopsy is a red flag as far as im concerned. there are neurologists who specialize in surgery such as nerve biopsies and this is what a number of them push for early in the diagnosis stage.
I was diagnosed with sensory motor axonal PN via emg/ncs and was referred to Columbia Presbyterian Neurology for a further workup. The first thing they wanted to do was a nerve biopsy. Thankfully i found the PN support site that was where the long term posters to this site were prior to going for the biopsy and as a result was tested correctly without the permanent damage which would have been done by a nerve biopsy.

If it were me i would do my research on nerve biopsies and if the doctor still refuses to tell you what he is looking for or suspects I would move on.