Sitting upside down like that might indicate a neurological issue, but I fear the only times I've heard of a ball python lying upside down is gravid females and death. He looks very thin, likely underfed and I agree with the dehydration worry. Good on you for getting him in the tub, hopefully that helps. When you set him right-side up in your hands, does he move? Does he try to move back onto his back? Also, he definitely needs something bigger than a fuzzy of any kind. What is his set-up like? Is the UTH controlled? You said Petco, right? What were they feeding him beforehand? Get him to the vet ASAP, hopefully they can help...It's always a toss-up when you buy from a pet store.