Baby is hurting me while nursing

I am wondering if anyone can give me some advice. My son is 3 months old. We have just recently worked through his latching issues, so it finally doesn't hurt anymore. That part is great. But it seems that now that we have that figured out, we have come to the next obstacle. My son has started to scratch the crap out of me while nursing, and he also seems to enjoy stretching out and then popping off of my nipple. It hurts so much, and I don't know what to do to stop it. At first I thought that meant he was done, but if I take him off he gets ticked until I put him back on. I know he is getting enough milk, because he is 3 months old and wearing 6/9 month clothing (and still growing quickly). Does anyone have any experience with this? Any tips on what I can do? It's frustrating me to no end. I'm starting to dread nursing him. 😞