What Is The Chronic Numbness In Leg From Toe To Armpit?

my son age 50 has a numbe left leg from big toe to arm pit. The other leg is very cold. Since last September 12th. He has passed had many tests to eliminate other things, so this is what they think it is. He copes and has had some physiotherapy and tries to keep active but gets exhausted and of course gets depressed. It came out of the blue. I would love to hear your comments and would answer any queries you might have. Thankyou.