What Causes Numbness On Right Side Of The Body?

About 2 and a half weeks ago my husband started having loss of sensation/numbness throughout the right side of his body which is now starting to affect his left side coming up his leg. He has seen 3 doctors and been to the ER twice and all his blood tests have come back fine. He is scheduled for a MRI on Monday but he is unsure if it will detect anything. There's no pain or other side affects, do you have any ideas