Our Dog is vomiting and not eating. What can be wrong?

Our dog started vomiting clear fluid 5 days ago and will not eat or drink anything. We took him to a vet who gave him anti vomiting pills and IV Fluid for dehydration and sent him home. He has progressively gotten worse. He is currently at the vet getting IV Hydration and anti-vomiting medication. All labs were normal except the sodium and potassium which were low due to the dehydration. X-rays were taken and normal. No Pancreatitis. They are going to perform a Barium Swallow today. Normally, he is a very active dog but now seems to be getting increasingly worse. He lays in the Vet cage and just opens his eyes when called but will not even get up. The Vet said if the barium swallow doesn't show anything then she doesn't know what else to do but keep him hydrated and treat the symptoms. I need advise. Any Vets out there? Is there something I am not doing?