D.,
If you're dealing with low supply and that's why you're supplementing, he could be biting down trying to get more milk.
I'm not going to tell you that if you just pump more, drink more water or take the right herb you won't have this problem. I had low supply with both my babies and took fenugreek, fennel, goat's rue (every herb I could find that was supposed to increase supply) and all the lactogenic foods available to me. I also had two different lactation consultants who helped me but finally admitted I just didn't have enough milk. I still had to supplement. Some women genuinely have low supply due to insufficient breast tissue or other hormonal issues.
So, whatever is going on that you're supplementing, good for you that you're still nursing.
If it's not low supply, it could be nipple confusion but if you've been supplementing the whole time, I consider that unlikely.
Try breast compression when you're nursing to give him a little extra.
Most of all, try to get some rest. If he's sleeping, SLEEP! We all hear this advice but not many of us take it. It really is vital to milk production and good overall health for you both.
Blessings!