My father will not respect my wishes to stay off the topic of the Dr. who diagnosed him with alcoholic dementia & losing his license. Advice?

Whenever I am with my 88 year old independent father (widow) he is relentless about getting his drivers license back and the conversation - no matter how hard I try to move off topic - keeps coming back. He keeps digging at me to converse with him about it; to the point that he is condescending, agitated, and blameful. I try not to reason with him any longer, try to change the subject, tell him I am not ‘going there’...to telling him I will have to leave if he persists. He is trying to be manipulative in getting his license back with doctors, wants me to write letters for him, and works hard at figuring out what he can do to sway them. It’s all he thinks about. I have shortened my visits with him. I work hard at making his day better for him, and it goes sour very quickly because he is stuck on it. It’s been 9 months. 2 Geriatric doctors later....he has no insight to understand why he cannot get his license back. I don’t want any part of it and it frustrates him as he says I am the only one who can help him. How do we move on?