Good that he's getting an MRI. That should help in the diagnosos. I doubt the shoulder injury had anything to do with it but you mentioned that he's had back trouble. Taking a guess, I'd say the first choice would be his back. Maybe a herniated disc. Many times herniated disks will go away all on their own in a year or so, so that would actually be a good thing. He may have did something more to it the fall that he took. It could also be something really easy (easy as in better than your back) like hypothyroidism. The MRI will rule out things like a tumor and also diagnose a back problem so he's right where he should be. It is a fairly daunting task to diagnose a numbness problem so he may need more tests also. Good luck and try not to worry, tumors or anything life threatening is a very very rare thing.