Was he a premie at birth - his hearing could still be forming . Was he used to loud noises from Day One? At five weeks his neck muscles may not be strong enough to turn his head on his own. Talk to him face to face, make funny noises to see if he responds with facial expressions. Also make use of a baby ear syringe to check gently for any built up wax in his ears. And, I would think that hearing screening at birth is difficult at best to gauge 100%.
Mine didn't startle to cry till she was 3 months. She did have the startle reflex, though. She still ignores me sometimes, but she has one toy that she can't stand the sound of, so I use that to make sure she's actually able to hear.