There are relatively few reversible/preventable causes at that age. This is just life. What he can probably do is eliminate nearly all practical impact from this problem.

To do that, get an audiologist-administered hearing test, then discuss the results with an otologist/ENT. Often this is done in the same office visit.

Among other things, the audiologist will test where the loss is occurring (Google "Rinne and Weber tests" or bone vs. air conduction), and at which frequencies. Get hearing devices tailored to those frequencies. The otologist will recommend options. These aren't likely to be all that expensive.

There's no real opportunity for greed or snake oil here. If the otologist's advice works, the results are literally audible.

This is not a difficult problem to almost completely solve, and it makes a huge difference in quality of life and healthspan: https://www.hopkinsmedicine.org/health/wellness-and-preventi... (Update: apparently this study was retracted - see reply)