Most likely your father will have two options, either to give his driving license up voluntarily, or to take a test. This is what happened to my father, he swerved to avoid a van pulling out and went into the back of somebody, no serious injuries, but the airbag went off and he had a few bruises, he was 77 at the time and had a clean license - he gave up driving voluntarily thank goodness. he was told if he gave up driving, no court preceedings would be necessary.
Can you contact 'Help the Aged or Saga, they usually have a team of volunteer drivers to take people out.