I recommend to transition from breast milk to Pedisure (high dense nutrient drink for extra calories and protein to help gain weight) is to mix the two. Start with ¾ breast milk and ¼ Pedisure, go to half and half, then ¼ breast milk and ¾ Pedisure, and finally to all Pedisure. You can do each mix every 3-4 days as long as he is accepting the mixture. He will get used to the Pedisure taste gradually. Once he is drinking Pedisure and accepting it, do not reintroduce breast milk at all. Suggest trying other dairy sources, such as yogurt and cheese, in place of milk.