I know for humans an obstruction won't show on X-rat and a CT or ultrasound is needed. I'm assuming it's the same for dogs. If he were human my guess would also be obstruction so I'm with everyone else. Not having any stool (only mucous) and vomiting are signs of an obstruction. I would get him back to the vet and have that checked. You don't want to let an obstruction go for long. Best of luck and I hope Baxter feels better!