My son was a preemie 31 years ago and had the same problem. We wound up having to put him a prescription formula where the proteins were predigested. He couldn't break down the protein in the formula milk or soy. It was very expensive but solved the problem. He was on it for a couple of months then went to soy formula. The pediatrician also had him sleep in a car seat in a reclined position vs. being laid flat. That also helped a lot. Good luck!