Dj, we had to deal with exactly that issue with my mother & father-in-law, unfortunately there is no good way of dealing with this. Best you can do is have a talk with your dad, tell him your concerns, expressing your fears about him hurting himself or others then ask him to go to the DMV for an eye test. If there public transportation services available in his area you should be prepared to explain those options to him. There is also Uber that will take him wherever he wants to go.
Both my mom & father-in-law were mad at us for awhile when we took the car but they got over it.
Good luck
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