My 86 Year Father Is Feeling Weakness In His Legs

My 86 year father is feeling weakness in his legs and loss of appetite or malaise. Up to a month ago he was exercising daily for an hour per day. a couple of months ago he fainted and was taken to the hospital where many tests were run. It was discovered he had had a mini stroke at some point in time but that was not the cause of his fainting. His family doctor had some blood work done to determine whether he has anemia but it was determined that he does not. What are some possible causes of weakness in the legs? Should he stop exercising completely ?