Based upon what my audiologist told me about getting used to hearing aids, your dad likely didn't give them enough time for his brain to retrain to interpreting audio impulses better.
It was 3-4 months before I got used to my hearing aids and didn't notice I was hearing differently as much as I did initially. My audiologist said to give them 1-2 at least before giving up.
I have mild tinnitus in one ear but was told to live with it as the tinnitus programming is a definite distraction unless tinnitus is so bad you can't stand it.