Ryancher, have you done the Ling 6 with your son? It will give you a basic knowledge of what he can and can not hear in speech.
You say "ahhh", "oooo", "eeee", "ssss", "shhh" and "mmmm".
Say them at regular volume, at about 3 feet away (make sure his back is to you). If he can repeat them all back to you, that is a simple test to show that he can discriminate speech sounds across all frequencies. If some are missing, you know he can't hear those.
You should be doing this each time his devices are put on to ensure that they are working properly and there is no change in response.
Caution: this is the very first step toward speech discrim. These sounds are very different, and people who easily discriminate them, still can have trouble understanding speech. There are sounds that are much more similar in English (mm vs nn or th vs f) This is just an easy way to tell if all frequecies are being covered.