25 days old preterm baby. Every feed?

My baby was born in 34 weeks and was weighing about 1.7 kgs at the time of birth, he is 25 days old now.. he is all fine but he was in NICU for 10 days as he was underweight.. from past 2 weeks, whatever feed he takes, mostly breast milk and express milk, even after burp and half an hour to 45 minutes after burping , when we put him to sleep, he vomits everything from nose and mouth, almost every time , and he keeps the milk in throat for a very long time making some noise in the throat.. when we ask doctors about his spitting and vomitting through nose and mouth always, they say it's normal and things will get better when he puts in some more weight.. my question is why is he vomiting always after every feed?