Bulldogs World

So our 8 1/2 year old bully has been throwing up on and off for over a month. It started at the end of October. He threw up about 2 days in a row, then he was fine and actng normal for over a week. Then all of a sudden he started throwing up everyday. We noticed it was every time he drank water and sometimes after meals. The vomit was always very foamy and had a lot of very thick mucus. We took him to the vet, they ran blood, stool and x-rays, all showed nothing and sent us home with 3 meds. He still ate everyday, went out side and bathroom habits are the same but the pills didnt help and he continued to vomit at least once a day. We called a bulldog specialist but they couldn't see him until the 28th. Last night he threw up over 7 times within 2 hours. We rushed him to the emergency hospital and they ran blood and he puked 3 times there as well. Blood is fine- they kept him overnight. When the vet looked him over, he was feeling around his belly/rib area and Baxter winced in pain and started licking and the vet said thats a sign of pain. Today they have done another xray and sonogram and still have not found a thing wrong. He has been on IV fluids and they even gave him some water and he held it down. Now he is having his throat scoped and they will call us soon. What could be wrong with him? He has severe allergies but we always feed him the same thing (duck and potato natural balance wet food) I called the food company and they have not changed any of the food in years. He didn't eat anything that he shouldn't have. We are at a loss and so heart broken that we don't know what's wrong. They also checked for pneumonia and checked his esophagus and all were good.