It is not exactly clear whether he has pain is the joints, or has rubbed the skin raw on his elbows to cause this pain. How does he indicate this pain to you.? You say the vet says it is OK, however have you had him x-rayed. Perhaps if you did, you might discover what is causing the pain. You should not let him sleep on cement or a hard floor, and should rather get him a big bean bag cushion to lie on, or an old duvet.
I would take him for another check up and explain your concern, to another vet.