Won't Breastfeed Well

My son is 3 weeks old, and for the first few days he would nurse just fine. Since my milk came in, he feeds less and less. He'll eat for a few minutes, pull of and start crying, then go back on and repeat this until he won't go back on at all. His latch is perfect and he's burping/peeing/pooping like normal, and he's putting on weight, but it's exhausting and upsetting for me. This nursing issue can go on for hours until he sleeps, but he sleeps for maybe an hour before we go back to this nursing issue 😞 Why is he not nursing well?