About then I had to start saying "You've got a big strong mouth - you need to open it or Mum gets sore". Show him the sore bits and explain that although you know it was an accident, he made you ouchy, and that he needs to feed properly and let go nicely!
And tell him when it happens - "you're hurting, try again", and also when he gets it right. "Well done - no toothmarks"
For us it wasn't any particular position - but it was definitely opening wide.
If he has a snotty nose then that may be part of the issue too.