In addition to looking at the science/evidence (WHO recommendations, biological norm for weaning, etc.) this is largely a *parenting* topic, *not* medical.  I have found some doctors & pediatric dentists who blend their personal biases with medical suggestions.  Naughty.  (this is one of my biases!)
You might let the doc know there are wonderful mothering groups available to help mothers find the balance for their own family, since breastfeeding does shift from (in his perspective) the medical realm to parenting.  Parenting topics are beyond his scope of practice, as his degree likely isn't in parenting.  
Amy Peterson, IBCLC

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