He and my Mom are in an ASF but Dad still wants to drive. Dad will be 91 in a month and has moments of lucidity but has prostate cancer and Myasthenia Gravis. He recently was in the hospital with Covid. Mom is 90 and has limited mental faculties. They are angry at my brother and I but there is nothing we can do. Now they have decided they want to have their car and go about as they choose. This is highly dangerous.
My own mother is 91 and has a tiny bit of short-term memory loss, usually mostly manifests at the end of the day (Sundowning). She drives to the grocery and department stores all within a 3-mile distance. She only goes when there is no bad weather and the streets are dry. She only goes out between rush hour/school busses. She has a cell phone on her at all times. Occasionally I will follow her (as secretly as I can) or have her drive me somewhere just to make sure she's ok. She is very healthy and has good vision, hearing, and motion in her neck and back (so she can turn to look). Most importantly she still has good judgement. She knows "someday" I will help her close the chapter on this activity, as I've done for 3 other seniors who were no safe to drive.
My long point is: just because your father is 91 doesn't mean it's "time". Maybe it is... but Instead of battling him with an abrupt end, arrange for their family, friends, neighbors, church acquaitances to take them to appointments, run errands, take a casual scenic drive. Give the volunteers a gc to a favorite restaurant so they take your parents out to lunch or dinner. The social aspect will have far more benefit and he will get used to not driving without realizing it. I did this for my aunts in FL.
It worked great.
In the meantime if he truly is unsafe to himself or others on the road, report him anonymously to his state's DMV (online). They will send him a letter to come in to get reexamined (eye test, maybe behind the wheel). Make sure no one takes him to this appointment and let his license expire. Or, intersect his mail so he can't act upon the letter. When my Aunt got hers she asked my cousin to drive her to the DMV so she could renew ;-) That didn't happen. I just made sure they had scheduled rides to anywhere. The only difference is there is not more "spontaneous" driving. And that's ok. Covid has helped limit some of this.
If he's very stubborn and unsafe, you will need to remove not just his keys, but his car. Make up a therapeutic fib ("the car's in the shop needing a very costly repair"). Park the car where he can't locate it or report it as stolen. Make sure all those he might contact know to NOT lend him their car.
To be fair, it's not just the loss of the freedom, it's what the end of driving (and other activities) means: one more step closer to the end of his very full life. I wish you much success in navigating this with your parents and peace in your heart.