How many oz of formula does your 6 month old drink a day?

Parents, I’m so torn and stressed now. My son was born 3lbs 3 oz premature and was in the Nicu for 42 days. He’s 6 months but on the smaller side. Our pediatrician has always pushed for him to eat a lot but he’s had acid reflux issues where he would throw it all back up and I felt like it was just to much on his tummy! She expects him to eat 30-32 oz a day in a 24 hour period. & 6 oz every 4-5 hours . He can only eat 4.5 oz every 4 hours and I can only get him to 27 oz in a 24 hour period! His formula is also fortified for extra calories which I feel like makes him more full also! I was giving him solids twice a day which she told me to stop. He’s 13 lbs, always happy, sleeps through the night ( even though now I wake him to get extra oz in him like 3 times in the night! ) and is hitting all his milestones !! She just says he’s not gaining fast enough. My husband and I are both 5’2 and his sisters are all way shorter then that so I don’t think my son is going to be very big. Please help and tell me how much your 6 month older baby drank in a 24 hour period & how much they weighed ??? I’ve never been so stressed and focused on something in my life! All the constant force feeding is making me feel like I’m giving my son oral Aversions ! He’s a very healthy and happy boy!