Hmmm, Don't know about anyone else, but the look in his eyes makes me think he is panting, from either playing too hard or not being able to breath for some reason? I don't know...it's cute....but it would worry me if one of my girls was doing it.
He doesn't appear to be panting. In fact, it looks like they came across him doing it and went and got the camera. They may have hit the nail on the head; I think it may be a "nursing," self-comforting behavior.
In fact, if you check it on youtube, that is exactly what is happening. The owner, who shot the video, said he suckles on the robe that he is lying on, but sometimes he forgets to keep it in his mouth.