My husband has had tingling, numbness and cold feet in his both of his feet for the past 2 months on a daily basis. What could this be?

Optional Information:
Male , Age: 41

Already Tried:
He has gone to all sorts or doctors...from internal medicine, to neurosurgeons, to neurologists and all the tests have come back negative. No one seems to know what is going on...the blood tests all come back negative.