I would look into Vit. D supplementation, as Vitamin D affects the ability of bone and teeth to mineralize and even remineralize.  My granddaughter had a similar issue, and I have seen articles that discuss how Vitamin D can affect carries.  
There is some research that shows that if a mother's Vit. D levels are adequate, that a breastfeeding infants levels will be fine as well, but the AAP recommendations still say to supplement the child.  
Kathleen

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