What Causes Constant Vomiting In An Infant From Birth?

Hi, My son is almost 14 months old and were in the hospital one again. This is our 5th stay and no one knows what is wrong with him. Since he has been born he has puked none stop. So because of him not keeping anything down he got a gtube. But that hasn t helped! He just hit 14lbs, he doesn t eat by mouth on through his tube. He has 2 wet diapers a day which is one when he wakes up and one before bed. He also poops once a week maybe twice if he is lucky. After puking for a bit his puke comes out brown. I was hoping if you had an idea, as I m losing my mind over nothing being done. Thanks, Hailey.