How do we convince our dad, who has Alzheimer's, that he isn't allowed to drive any more?

Our father was diagnosed with Alzheimer's two years ago and is still in denial that anything is wrong with him. Last Thanksgiving he got into a heated argument with our brother (his son) while visiting one of our sisters in Colorado. He packed up his things and his dog and drove home. He wouldn't answer his cell phone for anyone except for our sister in Phoenix where they both live. She would call him every couple of hours and ask him where he was. His descriptions of his surroundings were correct for the time frame he had driven from call to call. The last conversation she had with him, he was coming up to the exit on the freeway to his house. That was on Friday after Thanksgiving. We assumed he made it home okay until we got a call from one of his friends at church that he didn't make it to church on the following Sunday. We thought maybe he was tired from the long trip home. Our sister went to his house after she got out of church on the same Sunday and found that he hadn't been home at all. The panic set in as we decided to call the police. Just then, the police came up to the house before we had a chance to call. It turns out that he showed up in another town about 60 miles southeast of his home. He didn't know where he was and someone at the restaurant he had parked at noticed that he had been there a long time and looked confused. They called the police and they sent a specialist out to help him. He swore to the officer he knew where he was and that he was in Yuma but in fact he was in Tempe. The officer showed him his shoulder patch that said Tempe. After we arrived, the officer briefed us on the situation and told dad to hand over his keys to our sisters husband. We decided to park the car at another sisters house so that he wouldn't drive until he got the okay from his doctor. Well, his doctor said that he needs to give up the driving because he had been missing for two days and the police were involved in the incident. Ever since then his denial has become more insistent and that we have now stolen his car and that no one said he couldn't drive. Do we need to obtain documentation to support the fact that he isn't allowed to drive because of his mental condition that he only denies having? What can we do to convince him it's alright to stop driving?