These apps are found in the 'Health App' under 'Audiogram'. The app I used is called Mimi Hearing Test. Free, and calibrated for regular Airpods. There are two more apps listed for audiograms, Streaks and SonicCloud Personalized Sound.

Once there's an audiogram captured into the Health app, go to Settings > Accessibility > Audio/Visual > Headphone Accomodations > Transparency Mode.

Some of these are not immediately obvious as links. He can then modify the amplification and 'brightness' of the sound.

I'd urge you have him get real hearing aids though. They are pricey, but so much better long term. There are newer ones that can connect to IOS devices and stream sound.