Some little guys can take 14 inch dildos as big around as your forearm. Believe me, asses stretch out after a while. The deeper, gutteral moans are telling you he's really enjoying it. As far as the back hurting...well I can't help you there. I've had problems with my partners taking me but they usually relax after I fuck them for a few months. My BF of 5 years still has trouble sometimes, I always go slow.