--is likely to look for autoimmune markers and/or infectious agents--either which could cause a neuropathy, depending on what is found.

It could also come out clear as a bell, though, without eliminating the possibility of autoimmune or infectious factors at work.

There are some paraneoplastic syndromes that produce neuropathy through autoimmune mechanisms--and many of the blood cancers (lymphoma, Waldenstrom's, myeloma) can have neuropathy as a concurrent or presenting symptom. I would assume (though maybe I shouldn't) that if he's gone through at least a bit of a work-up, he would have been checked for obvious signs of these--have they done a peripheral blood smear and cell typing, and an immunofixation electrophoresis of serum and urine (to check for the monoclonal or M-proteins associated with blood cancers and neuropathy)?
This reminds me--always good to put in a plug for getting copies of all of your and your husband's test results, and to organize them using the Liza Jane spreadsheets:

www.lizajane.org

Much easier to track results/patterns and to suggest other possible tests to physicians who otherwise might not think of them.