I'm replying purely to try and reassure you as we had issues with DD's hearing that ended up being nothing. If he passed the newborn hearing test and has had no problems/infections with his ears then that's a good sign.
DH and I were out one day and it was windy and stormy and a big metal sign blew over. We jumped out of our skin and DD didn't even flinch. I think she was about 6 mths old. Even if we clapped out hands behind her head or shouted behind her she wouldn't react. Fast forward we went to a ENT specialist who did a hearing test and told us that is was likely she had mild to moderate hearing loss and an appointment would come for further testing and the fitting of hearing aids. This threw me as she'd passed her newborn test and I was convinced I was worrying about nothing. A mth of stressing passed (I should add that if she needed aids that's fine, I could cope with that, but my sister has a genetic condition where she wears hearing aids but is slowly going blind). As soon as I mentioned that they started talking about genetic testing/MRI's etc but as time went on it became apparent it couldn't be tested as they have only identified one of the chromosomes that causes it - they could eliminate that one but not the other so it would be a case of wait and see. I was a mess and could barely hold myself together...wait until she hits late teens/early 20s and watch and see if she starts to go blind. WTAF
Anyway, went to the appt a mth later and she passes with flying colours. Turns out the testing that was done earlier was age specific and she was too young for it. I was absolutely livid no one had told us this.

My point is if you are worried go and get a referral but chances are he is fine. I noticed with DD that often they get so engrossed in things (even a hand can be fascinating, what's this, oh its my hand) that they just don't seem to hear noise. Its not that they cant hear it, more that what they are doing is more interesting to them than looking to see where something is coming from. If he turns to look for his favourite commercial that's a pretty good sign

But get him checked out for your own peace of mind otherwise you will always be subconsciously checking (trust me on this) and at time you will drive yourself mad xx