In the end it doesn't matter what he was doing when his son was left in the car, right?
Well not for most things, but in this case it means he's pretty much screwed even if they don't charge him with anything related to his kid's death.
That, and in cases like this, whether and what he gets charged for tends to be based entirely on how likable he is. It shouldn't be that way, but that's how it ends out. If he appears to be a fine upstanding person, we feel sorry for him, and the consensus is that the death of his child is punishment enough. If he's not so fine and upstanding, no matter how irrelevant that is to the death of the child, we wont feel as sorry for him and will be much more willing to demand that he pay in some way. Consciously or not, we (as a society) tend to act as though if the parent is sinful, then the death of the child was some kind of punishment for those sins, and thus the parent is responsible. It's completely silly, but that's how these sorts of things seem to work.