He's referring to a specific point in time: When he arrived that day.
We don't use the present perfect with a specific point in time.
When I arrived at 8:30 this morning, I didn't see your car.
* When I arrived at 8:30 this morning, I haven't seen your car.
If he was referring to an extended period of time (I haven't seen your old car over the past several days. Did you get a new one?) rather than his specific arrival this morning, it would work.