I appreciate your point, and perhaps it was just a bizarre (IMO) word choice in a terrible situation. At least according to him, he didn't realize he'd done it until he was driving and then he quickly pulled over and got him out of the car. Agreed there would be an element of time there for him to dread getting a clearer look at his son from the time he "realized" he left him in there to the time he could pull over, so I take your point.
Nevertheless, I think it is a very strange thing to say to the wife. Who really cares what he dreads/dreaded? Why did he care? He had, on his own account, just roasted his son - and his attention should have been at least on his guilt from having done that as opposed to whether Cooper's appearance would be disturbing enough to be dreaded. Likewise, his wife's concern on whether he said too much is a great legal concern, yet a bizarre topic under the circumstances. Likewise, his concern over his job and possible punishment when his son's life just ended based on his own actions, negligent or intentional.
I just don't get his behavior in any respect. Another example, given that I think he did it on purpose, is cursing the police while he was on the phone. Why do that if you want them to believe it was an accident - why antagonize them at all? That really seems like a reaction of desperation, which would lead more to the innocent side, though I don't really think it indicates that.
So, anyway, digesting your point I think none of it amounts to a confession. But, it all comes together that so far as we know his expressed thoughts to his wife (and maybe there's more to come on that) were more concern for RH's future, and minimizing Cooper's loss (he was peaceful or whatever) which to me is similar to his wife's attitude in her funereal comments, where she said she wouldn't bring Cooper back if she could yet she was "doing it" for RH and perhaps they'd have new children.
Worst to me was watching him during the hearing, during which he showed no reaction that I saw to testimony about Cooper and his death, yet seemed attentive and alert during timeline statements and the like that I suppose he thinks he will be able to torture into saving his hide.
The guy is flat-out bizarre, for certain.