A:Answer As a grown man that plays shooting games around 4 times a week, I can tell you that that is NOT TRUE.
One of the following is happening:
Your son is lying and doesn't want all his audio in his ears because it makes it harder to have a quiet voice chat channel (with the headphones)
Or, your son doesn't know that the Playstation4 (this is a PS4 headset, so I am assuming he plays on ps4) has voice settings to mix or split voice chat and game audio.
also, most gamers do "party chat" and there is yet another audio mix configuration for party chat audio.
to fix it, go watch this youtube video: https://youtu.be/mdqHkRaGLjw
ENJOY THE SILENCE