Help!!!! EBF to pumping!

Hello ladies.. my son is 4 weeks old. He was born early at 35+3... I’ve been EBF since birth. He wasn’t gaining weight for two weeks, wasn’t losing either but they determined he had a dairy allergy so change my diet, keep BF & give a little nutramigen to speed weight gain... all that said I’ve started pumping because I can give less formula if put in breastmilk instead of water however I’m only able to pump about 25mL from each breast every 3-4 hours... shouldn’t I be getting more? I mean shouldn’t my baby be eating more than about an ounce & a half at this age? Or no since he’s a late term preemie & still only 5lbs 10oz?