I exclusively breast pump and im worried my son will eat more than i pump?

My son started out in the nicu and I was not able to breast feed him there so when he got home he would not for anything latch on or swallow if he did. My pediatrician said its pump or formula because he wasn't gaining weight. So I pump after feeding him every 3 hours for 20 min on the medium setting. I usually get 3 oz but he is starting to want more and I am wondering how I get more. The higher settings hurt but if that's the only way to get more I'll do it. Will my body naturally just produce more? Oh and my son is 5 weeks...