Where do I start?

Welcome. Sorry you need to be here. I hope you plan to take the keys away. Loosing the independence of driving is difficult and many/most with dementia don’t recognize their symptoms or lack of skills. They often see no reason for a loved one to take independence away (even if they know they have been diagnosed with dementia) making them angry and upset. How is his memory? Will he remember the diagnosis if he is told? If he doesn’t remember, doesn’t accept the diagnosis or sees no rush to plan for the future, I would not push his acceptance. Do what needs to be done (will, DPOA, take away car, finances etc) without making it about his shortcomings or dementia. Suggest that a will and a DPOA are just something that a person of his age should do, disable the car or “bring it in for repairs”. I know this approach seems dishonest, but it may be the lest stressful for him. I my opinion that’s more important than honesty. To back track a bit, seeing a lawyer and getting a DPOA is very important, I would not wait any longer than necessary on this. It sounds like it might also be time to take over finances. This is often one of the first signs of a problem with dementia. I hope something here is helpful.