I'm new to forum but thought I'd let you know I was in same position with my father. I sat down and said I didn't think his responses were quick enough for driving and his general movement of his body was not quick enough and that he had to think of other people and children, rather than just himself as he he didn't need to be travelling very fast in order to hurt or kill someone by driving. At this point he said he thought he was okay and a safe driver.
I then rang the doctors to see if they could stop dad driving as I didn't want to be the one to stop him which is probably how you feel. The doctor said they could not stop him but I asked if next time he had an appointment could they mention driving and discuss the need for him to think about the future and that sooner or later he would need to stop. About a month following I took my dad for a routine checkup and the doctor did mention driving and said my dad needed to think about the deterioration in his mobility and he should consider giving up driving. We got home and my dad said yes the doctor was right, it was time he didn't drive. Success or so I thought but I later found out he did drive another two times but then did stop and I am now in the process of selling dad's car.
I think sometimes it just take an outsider to put their view forward to help assist the sufferer to make the right decision.
I hope this helps x