Hi, I would like to help you with your problem.
From what you have said, it sounds like your son is simply experiencing a very common phenomenon that most of us experience in our lifetime. It is not uncommon for people to think they have heard someone speak to them when they have not. Hearing your name would be an example of this occurring, especially in a group setting where the possibility is much greater. Your son may have simply been hearing words in the group that were close to the sound of his name.
Once your son has experienced this phenomenon once or twice, he may be more in tune with it occurring again, actually paying attention to it happening. This may explain it occurring more than a few times.
In the absence of any other symptoms, I believe your son is fine. If it would rest your mind, you could speak with his doctor to confirm that your son is ok.
I hope this has helped you. If so, please accept my answer so I may receive credit.
All the best,