Breastfeeding...

12 day old chews on my nipple. I know he gets the milk due to heavy flow but no matter how much I try he won’t gum on the areola

He did at the beginning but just for the past 2 days he’s getting very lazy with feedings and just latching on to the nipple and when I do get a deep latch, 2 mins later he’s slipped back down just to the nipple

Is this bad? How do I resolve it?

Also, how do I know when he’s had enough. He stops and starts feeds a lot....