There is physical therapy available for adults with vertigo and balance issuse, these may help your son, especially if the vestibular system is involved. You could have him adjusted by a Chiropractor as well. This can help his central nervous system (brain, spinal cord) communicate with his body better so it has better control over his balance. Can't hurt, may help. I could recommend someone to you, if I knew where you live. I used to practice in Bucks County and know a few DC's there and in the Paoli area (I now practice in N. E. PA). Another suggestion - has he seen a pediatric neurologist? CHoP and Geisinger Hospital are good resources. They may know of a diagnosis/treatment a neurologist that normally treats adults is not aware of.
good luck, this must be very frustrating for your son.