HIPAA laws are very strict. A relative may give info to the doctor, but the doctor may not give patient info to anyone unless the patient has been declared incompetent and a legal medical power of attorney has been assigned. There is a difference between a power of attorney and a person legally assigned to make medical decisions for a patient. Just because the doctor is 76 doesn't mean he shouldn't be practicing medicine. As an RN it sounds to me like the doc was following the law, and maybe he chose to let your dad go through the process of taking the tests, etc. If he couldn't pass, then that would be the end of it.
When we became aware my mother's declining driving skills we contacted the DMV. They sent my mother a letter saying because of her age she would need to take a driving test. She knew she wouldn't pass the test so she quit driving. One of her hands was very weak and her eyesight was poor. It was all anonymous, thank goodness!