I Am Checking To See What The Possible Causes For

I am checking to see what the possible causes for an 84 year old male suddenly unable to bear weight on either leg. He is able to lift, move and bend them at will but when he goes to stand, they are too weak to hold him up. There is no pain, numbness or tingling. EKG, Cat scan and blood work all normal. There are no other symptoms.
He has been admitted to the hospital but they are at a complete loss