Have child tip his head back more, sounds like he has his chin nearer his
chest and so weight of head is digging upper teeth down into the breast.
Review feeding position overall, head position and body position.  If this
child is sitting up to feed he may be too long in body length to have his face
right opposite breast so he is forced to tip his face downwards to take up the
breast.
kind Regards all
Helen M. Woodman, NCT Breastfeeding Counsellor UK