It's been awhile since I had a nursling, but I do remember my ds1 doing this. The advice I was given really worked. I would brush my nipple against his lower lip, and wait until he opened his mouth really wide. Then I would bring my breast to his mouth instead of him moving his head to latch on. I don't know if he was just so excited to nurse that made him lunge and clench when he latched on, but that did seem to work.
My kiddo does that sometimes too. He's 9 months. I have found he does it most when not really paying attention to how he's latching. I try to get his attention and say something like "gentle" or " no biting". That has helped a bit. Also when he does bite, even in the very beginning, I'll unlatch him and say "no biting!" and wait a second before latching him again.