interesting case... i would definitely clear the somatosensory system-- make sure his feet are still sending good signals up... no diabetes or other neuropathy? check the dix-hallpike again-- move him quick and relatively "deep"... i once had a lady who wasn't demonstrating a positive with my student until i took her down just a little faster...
if it's not BPPV but a non-descript vestibular balance disorder, standard balance training removing or decreasing somatosensory input would hopefully help him out... plus increasing the use of his somatosensory system and balance reactions should be good...
again, good luck with it!!