At that age they can't test the hearing as such, but they can test that the sound signal is getting to the brain. It can be done even while the baby is sleeping. This is done to all nbs in Qld (where the parents consent). If you have had this test already, there is not much more they can do just yet. But if you haven't, it might be worth it to give you some peace of mind. As I say, they can't tell you for sure that he is hearing, but it rules out some of the problems which lead to deafness.

I hope this is nothing for you. If it helps, I have heard of this before - I even think I was worried about DS1's hearing in the early days for the same reason. And he hears just fine (too well actually, he doesn't miss hearing or repeating anything LOL!). So there is most likely nothing to worry about. The check is probably for your peace of mind more than anything, and also so that IF there is a problem, it can be treated early.