28 weeks 3 days preterm baby weight gain

Hi, I gave a birth to twin babies at 28 weeks 3 days of gestation. Baby1 boy birth weight 1.066 kg and baby2 girl birt weight 980 grams . Unfortunately baby 2 we lost after 3 days of birth due to heavy infections.baby boy is stable and no digestion problems while taking 16 ml feed for every 2 hrs and moving hands legs body all are good. brain and heart scan also good. But he is  weight progress is very slow like 2 or 3 grams a day. Suddenly 10 days back 20 grams decreased and doctors identified medium urine infection, medication used and now it's under control. Now baby weight is 1100 grams in 33 days he just gained 36 grams more then his birth weight. After all healthy signs and symptoms also his weight gain is very very slow. Now baby 32 weeks 5 days gestation but every day weight progress is 2 or 3 grams. Please let me know the where is the problem for his weight gain ?