Reduce intake of feed of my baby. Into?

Hi Doctor My son is 6months he was 33weeks when born  and weights around 6.75kgs now. He is under express feed since birth and his intake was around 700 to 750ml. Since 2days my son intake has reduced and he gets irritated during the feed. We have started the formula milk from this month along with express milk. He is active but his urination has reduced during night. We kept a watch on his urination and he seems to have no pain. My concern here is he was putting weight till his completion of 4th month. In the 5th month he has put on only 300gms. His birth Weight was 2.3kgs. Me and his mother were covid positive in the month of May but have fully recovered now. Can you please suggest if there is a concern in this kind  and is there anything that we need to look into? Thank you