My father has some dementia stemming from a hematoma and a previous stroke. He still lives alone and drives his car. He has no issues with directions or where he is. He has trouble with dates, counting money, and is getting worse pretty fast about telling old stories over and over that are not accurate date-wise..etc. He just turned 87 but insists he is 88 and gets mad if we try to correct him. We have no elder dr. in our town. His reg. Dr. tried to tried to talk him in to not driving, but he just got mad and went on a story telling binge. My brother and I are his only children. He is starting to lose his social skills and its embarrassing to be with him in public. How do you know when it's time to do something with him and what? He absolutely will not let someone live/help him at home.
Take the car away ASAP. The next question is, can he manage his medications? Probably not if his memory is that badly impaired. Pursue Guardianship with the intent of moving him to Assisted Living.