How do I convince my parents to give my POA before it’s too late??

My mother had a massive stroke 9 years ago and is physically and mentally impared. My father, a lawyer who still practiced law successfully (then in his late 70s) then became her primary care given. Last year he was diagnosed with dementia and despite my begging him to seek professional legal assistance WITH me to discuss a POA, he has refused. I believe it's a control issue because he is afraid of anyone else making decisions for him (there is only myself and my older brother who unfortunately is a tragic alcoholic). The situation is degenerating rapidly and I know if something does not change very soon, I will have no choice but to ask for guardianship. Any ideas on how to speak to my father? Thanks.