Vomiting in 3 month old preterm. I do?

My baby  boy  has vomiting since 1 week of birth especially after taking formula feeds ( delivered prematurely at 36 weeks ). After stopping formula feeds his vomiting stopped. Doctor said probably due to cows milk allergy and told me to avoid cows milk too.but he has vomiting  and still continuing - sometimes milk like , sometimes curd like. But he is not vomiting when he sleeps . When he is active he vomits / spits up intermittently . His birth weight was 2.2 kg and he is gaining weight adequately.  now he is 5 kg and 14 weeks now and corrected age is 10 weeks. What should I do ? Tried different methods like burping for 15 - 20 min , head elevated position etc. Still he is vomiting . When he sleeps no vomiting at all. I'm worried what should I do