Vomiting every night with cry. Help me.

Dr, i have a 2 year n 4 month baby boy born in 34th week with weight of 2.1 kgs. Since birth we noticed acid reflux n condition was same till 1 year n 2-3 months. Though i manage to gain him weight decently.after that we noticed remarkable change in the condition n we was almost without vomiting except late night with loud cry. But from last 20-25 days he is vomiting 3-4 times in a day (almost after every meal) he vomits what he eat 3-4 hrs before. He vomits every late night( he sleeps n get up with cry n vomit). Right now he is also suffering from loosemotions but that is not my point of concern. I like to stop these vomitings completely bcoz he loose 2 kg weight in last 1 week.i m still wondering y always night vomiting even after 4-5 hrs of feed. Kindly help me.