Feeding issues for preemie. Is it fine?

Hi dr. I have delivered baby at 31 weeks 4 days.He is 35 weeks now . He was in nicu and since then i have been giving expessed breast feed . Earlier , the milk was sufficient as i was expressing in every 3 hours . Now , due to increased demand, i am expressing in every 2 hourly and giving it to him . But if he poops and peeps , his stomach becomes empty and takes the feed in every 1 hour . Sometimes ,he doesnt take 3 hourly , sometimes he takes it even within 1 hour. The routine is not setting properly. I am really confused as giving within an hour affects his stomach because of over feeding or is it fine ?Right now,he is taking within 45 -50 ml each time and after pooping , and he is taking again sometimes 30 ml, 10 ml within 1 hour. If i am giving him directly , i am unable to track the quantity and time as he sleeps a lot and doesnt know if he is full or not after half n hr direct feeding too. Please help dnt know how to feed him properly with proper feeding intervals and schedule.