What Causes Infant To Vomit After Eating?

I have a 4 month old who weighs 13 pounds currently. He was 35 weeks according to the abgar test I was supposidly 37 weeks when he was born, He swallowed merconium when he was born via C-section, he was put on antibiotics, he had fluid in his lungs so they put him on oxygen and tube fed him. He throws up anything he eats at random times during the day. For example, he will eat a 3 oz bottle thickened with 6 teaspoons of rice cereal sometimes immediately after he eats he will puke or at times he will sit for a while and pick him up and will puke or at times 2 hours after a bottle he will puke. I have seen a pediatrician about this and we tried 5 different formulas, he threw them up. He then referred me to a pediatric gastroenterologist who told me 3 oz of formula with 3 teaspoons of rice cereal. He still puked but went from 12 pds 4 oz to 13 pds. He told me to thicken 3 oz of formula with 6 teaspoons of rice cereal. He doesn t want to do any tests on him, asked him about the barium swallow or different tests. I have tried 5 different formulas, thickening the formula with rice cereal, feeding rice cereal out of a bowl with a bottle of milk. I now have to give 4 oz of prune juice due to constipation with the added amount of cereal to bottle. I don t want him to puke or anything, I feel bad he is doing this, What can I do?