Your dad is lucky to have such a concerned daughter. Since this behavior is long-standing, it is unlikely that he is suffering from some kind of dementia. However, there are many possible explanations, both medical and psychological, for the disconnect between what he experiences and what others observe.
There’s a limit to what a teen can do. I hope your mother is in the picture. If not, I really hope there is and adult in the extended family you can reach to for help. He is more likely to listen to another adult than to you.
Your dad would probably benefit from seeing a neurologist. If he is medically fine, then it is just possible that you are seeing behavior that is connected to either the autism spectrum or a personality disorder. I can’t definitely say one way or the other. He should be seen by a qualified mental health specialist for an evaluation.
Since your dad is disconnected from understanding the effect of his behavior on others, ideally, he would give you and other members of the family permission to share your experiences with the doctor. Once he has a thorough evaluation, he will be presented with a conclusion and a plan. It is then up to him to decide how to proceed.
I very much hope that someone can persuade him to have an evaluation. He and the family are missing out on the relationships he could have with all of you.
I wish you well.