Here's something I am puzzled about....did they own TWO car seats?
Those car seats are not inexpensive and many young families have two cars but one car seat. So the parent transporting the child, if they do indeed "take turns"....has to move the car seat to his specific car. The presence of the car seat...taking up a sizable part of the back seat...is then a big clue...that YOU have the child with YOU!
How did he lock the car, facing it...or come back at noon and not have the sight of that car seat remind him...that HE had the child that day? In a small SUV, these car seats are not inconspicuous anywhere.
In any event, if they took turns, then he has the physical act of actually inserting the car seat from her car to his...then strapping in his son...then driving, then removing him at the breakfast stop, then strapping him in again, then driving a just few more minutes...and, oops, forgetting.
Then he approaches the car again later in the day. Even if he does not see the child, you cannot miss the car seat in a small SUV. Then he actually enters the car and begins to drive home. I wonder if he had to back out of the parking place? Turn your head or look in the rear view...there is the car seat! Again, how does the presence of the car seat just blow past him?
I am having trouble with a father being this distracted under these circumstances for this long.