To be fair, it's hard to judge anything from a photo. It's like, what, one second in time with no context before or after!
I am not defending his actions and I believe this was planned (leaving Cooper in the car), but with this photo - maybe Cooper farted or needed a diaper change right before, which caused a less than enthused expression on his face. Or maybe she said something that annoyed him. Or maybe he had a go to the bathroom really badly after letting Cooper sleep on him for awhile and was going to get up, but she was all, no wait, just a moment let me take a pic and he was annoyed more with her than with Cooper. Maybe she accidently made too much noise and so we was giving her a look as if to say, shhh, he just got to sleep and she took the photo capturing his expression.
My point is, we have no idea what was going on before or when this photo was taken and I sort of think it's grasping at straws IMO, to read so much into it. If there was video, then maybe I could understand, especially if it was longer than a few seconds, but a photo? It's too hard to know what is going on. I know I've had countless photos taken of my where I look sad, angry or whatever else when I wasn't because the camera merely took a snapshot at a particular second in time where my face gave the appearance of those emotions.
So me personally, looking at this one photo and reading so much into it is like trying to read tea leaves. IMO.
I wish there were some home videos that were leaked or posted on social media. I think those would be a better judge of his demeanor and behavior with Cooper.