While this is obviously something he should see punishment for, it does sound like a mistake. There was a Radiolab or This American Life story about parents who forgot their children are in the car and the same thing happens. It's incredibly upsetting but from the article it seems like he didn't leave him in there intentionally. I don't know man, he should face some repercussions but from the quote where he states he only realized what happened when he saw that Endy wasn't in his kennel... sounds like he made a mistake.