The city around him is only built for driving, doubly so his completely car-dependent neighbourhood. He'll be doing this until he either cannot possibly pull it off, or gets in a serious accident. This is a fact of life for millions of seniors in North America, and saying "oh no they shouldn't" won't change squat.
No it's not "my job". It'll be his kids' job when he's fully senile, but until then he's an adult human being with rights, and he wants to be independent and "age in place". At some point he'll cross the threshold into having his decision-making powers taken away. Let's hope no one gets hurt in the meantime.