EP ftm!

So my son doesn’t latch, even with nipple shield! I’ve tried everything & that’s perfectly fine that he doesn’t. I’ve been pumping breast milk & bottle feeding.. but the past two days my supply feels slower than usual. A few days ago I was able to pump about 4oz per breast in 20 minutes. Now I’m only pumping barely 2oz per breast in 30 minutes. What can I do to increase my supply? My nipples are killing me from constantly pumping!