Dad rapidly loosing hearing

My dad has had to have a lot of testing done because he is rapidly losing his hearing. All of the tests have came back normal including MRI and CAT scan. The doctors don’t know why he is losing his hearing so quick. They said they’ve never seen this issue before. Unfortunately at some point he is going to be going to be deaf. My dad just said it will be awhile before that happens. I’m worried he is going to be deaf within a few years. And with hearing issues I think a lot of people end up with Alzheimer’s or dementia. I think a lot is going to change for our family and the next few years unfortunately. He is only 67. I actually had a bad dream last week that he was going to be deaf and now I know it is actually going to happen.