I'm not an audiologist, but i do know a bit about the problems you're describing...it's not the overall volume that is the issue (generally) but actually the decline in being able to hear certain frequencies. (specifically in the speech range)
an appointment with an audiologist would give you the test results you need for hearing aids that can be easily adjusted with individual frequency notches...(most with an iphone app) to get him to a comfortable place...
also, good hearing aids cost real money...
hope this helps .
"One POSSIBLE future. From your point of view... I don't know tech stuff.".... Kyle Reese