Dog throwing up with stomach pain?

I have been bringing my dog to the vets for about 5 months now, trying to figure out what is wrong with him. I have tried two different vets and I'm on my third now. My dog has a lot of stomach pain. He lays on the floor and he will growl for no reason or bark really loud, then jump up and run around till the pain stops. I asked the first 2 vets to look into it and they just said that it was an enlarged prostate. So we got him fixed and it seems like it's worse now. I went on a 3 hour drive to see my parents and he couldn't stop barking and growling from the pain the whole way there. Now he is throwing up bile/stomach acid at least 1-2 times a week. Help me. I don't know what's wrong with him and I feel like if I have a better idea then the vet will listen to me. I have looked for signs of flea's, works and stomach ulcers, but I can't find anything wrong with him which will make the vets investigate!