I have the exact same profile of hearing impairment as your son. I also wear hearing aids in both ears. One of the reasons that I love traveling to WDW is that the audio in rides and shows is LOUD. I can hear and understand voices and song lyrics in a way that I can't elsewhere (I usually use captions to watch tv). I've never felt a need to seek out a listening device.
The most difficult thing for me is time in lines- especially single file lines. If my travel companions are ahead of me, so that I can't see their faces, I struggle to hear them. Be mindful of that and make sure you are facing your son when speaking to him. This is true anytime in crowds.
Other than that, I find Disney to be a great place to soak up wonderful sounds that I often miss out on!