Pumping for premature baby, supply decreasing. Please read

My son was born 7 weeks early so I’ve been expressing milk for him to feed through his NG tube. He is 3 weeks old now. My supply has consistently dropped for the past 5 days and I’m now struggling to keep up with his daily quota. I think a big reason for the decrease is stress - I am definitely drinking enough fluid, although I could probably eat a little more than I have been - I also pump every 3 hours for 15 minutes routinely. Sometimes I express for an extra 5-10 minutes to empty my breasts a little more because I know that supply and demand is important. We have recently been using a nipple shield to latch and my son has had small feeds at the breast, but it doesn’t seem to be helping with expressing enough milk. I know that baby is more efficient than a pump, my supply might be ‘evening out’, or maybe my body’s just not responding to the pump effectively. But I’m just wondering if anyone has any tips about expressing milk because my boy is still mostly tube fed, and I’m struggling to give that to him.