Hi, do you drive? If so, you should take over the task. If not, consider the risks of him getting into an accident or causing one. How would you feel if he killed someone and you didn't do anything to prevent it. Or how would he feel? I know this because...my father in law has alzheimer's and we had to ask him that same question a couple yrs ago. He was a bad driver and did not realize it.
We live in Canada and maybe laws are different, but we simply called the motor vehicle branch and they sent him a letter saying he needed a medical, eye and road test done. If he passed, he could continue, if not his licence was removed. He just decided to turn in his licence. We soon sold his car as it was a sad reminder for him. Now he is fine. Too bad the disease is a lot worse though. Almost time for a home.