Maybe there's something wrong with her udder?
I would try to gently feel all over, and see if she flinches or acts like it hurts.
Maybe milk out a little bit of milk from each teat (see if that hurts her), and look/sniff to see if it's normal (because if the milk is not normal, that also indicates something wrong with her udder.)
Check if he's suddenly grown teeth, or has any extra-sharp edges on his teeth.
Make sure the mother has plenty of water available at all times, because if she does not drink enough, she cannot make enough milk, and that could cause him to pester and her to kick.