I went through this with my Dad when he was about the same age several years ago. My siblings and I had been warning him for months that he needed to give up the car.
Then one day when I met him for lunch I noticed the car had a broken passengers side mirror, and the passengers side of the windshield was shattered. I asked him about it at lunch, he dismissed it as something that must have happened to his car in a parking lot. I could tell he was a bit uneasy. So after lunch I told him to fess up, that I didn't believe him.
He then tells me he thought he had hit a pedestrian while he was going down the road. Thought he saw two people in the curb and one of them was snatching the other out of the way. Needless to say I freaked, then the siblings freaked. I asked him if he stopped. No. It is a busy road, so I surmised that had he done what he thought somebody would have tailed him and notified the police. We called his lawyer and told him the story. Lawyer called the police to see if anything was ever reported at the specific time, date and location. Nothing reported. But that was it. He didn't like it, but that was the end of his driving career.
He's held on to his old car that has been parked now for years and finally ready to get rid of it. My Mom has since passed and her car is still at his house; family members use it on occasion, he won't get rid of it simply due to sentimental reasons. At 96, he is limited to occasionally breaking the house rules and manages to knock the belts off of the riding mower.....the next step is to tell him the old mower is in need of replacement and do what other friends in the same situation have done---get a zero turn mower that he can't operate.
All that being said, in your case since your Dad has let his license expire, you might pay a visit to the local DMV beforehand to make sure he fails the eyesight exam (wink wink) and maybe that is a good way to handle it.
It's stuff like this that can really hit home:
http://www.wapt.com/news/central-mississippi/man-charged-in-fatal-hitandrun-dies/27074962