[car]

Not sure if this is a solution for you, since you describe neural processing fatigue. The Apple AirPods Pro have the ability to modify sound output based on an audiogram. The audiogram can be generated by other certified apps, that have been calibrated, on the regular AirPods for example. It is then stored in the Apple Health app. It is pretty amazing, wearing the AirPods in this mode is almost as good as hearing aids. But I am not sure if it can address your neural fatigue, for which you might want to get a second opinion.

[joerick]

Have you got a link for one of the 'certified apps'? I couldn't find anything online but I think my grandad could use something like this

[car]

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.