I think that is exactly what he wanted to happen and sooner than later. Hence the "how long to die" research. (He knew death would come fast) It didn't happen though. He checked at lunch. Waited some more and hoped someone would see the baby in the back. Again nothing.
Then he panicked. Drove in a stinking reeking car trying to decide where to pull over and make his dramatic discovery in front of witnesses, where he could put on his show.
I do not think that what actually occurred was ever part of his plan. I think he wanted the discovery to be made, by someone else, in his work parking lot. Then he would be notified and could have his breakdown right there. This poor father.....sitting at his desk for an hour and totally forgot his son in the car and he died. How tragic! Etc. Etc. Etc.
That plan ^^^ would ( in his mind) make his "innocent" forgetfulness more believable than 7 full hours of forgetting and then driving around for 10 minutes in a car that stunk to the high heavens. I believe this is where the "my son is choking" comment came from....he was panicking as he had no plan B and was forced to improvise at the end of his work day.
If you think about it, everything that he does after 12 noon makes him look the most guilty. The going into the car....not noticing Cooper. The getting back in the car at 4 and driving for 10 minutes in a putrid vehicle, he didn't smell anything that tipped him off, instead a "choking" (dead) cooper alerted him that his son was in trouble. Without the events from Noon on, we only have a father who "forgot" after sharing a meal with his son at Chikfila.
His original plan did not work.
That is just my opinion.