Capacity issue?

My little man is 15 weeks old. He was born 8 lbs 1oz but dropped down to 7 lbs 2 oz at four days. We started supplementing with formula b/c weighed feeds showed that he wasn't taking in very much. After a week pumping after every feeding it looked like things were on track and he gained 0.5-1 oz a day for 2 months. However between 2 and 3 months his weight really plateaued and he gained a little less than a pound. The pediatrician was concerned that he was dropping off the curve and suggested that I pump in the morning after my first feed and give it to him in the evening as well as making sure he eats every 3-4 hours at night (he was giving me 5-6 hr stretches.) His weight continued to plateau and I started taking herbs, did a weekend nursing vacation, and spent about four days pumping after every feeding along with power pumps at night. I have a home scale and after the four days of pumping I saw an increase in the amount he was taking in at each feed. I cut the pumps to 3 a day. However, his weight gain never went up and the amount he's taking in dropped back to where it was. After all this I began supplementing with about 2-4 oz of formula a day and now I've transitioned to supplementing with about 3-4 oz of donated milk. He's been gaining with both but it can be a struggle to get him to take it. I'm trying to give the supplement right after meals so that I don't kill my supply. I'm not sure what to do at this point. He's taking in more than an oz an hour during daytime feeds which seems to put him in the normal range. Is this a capacity issue on his or my end? Help!