I wouldn't worry about the lack of formula if he is eating yogurt. You get all of your calcium just the same. Make sure it isn't heavily sugared or have artificial sweeteners in it. The baby ones are really nice now. Have you tried giving him the formula in a sippy cup? Sometimes that is fun. You could have your milk in one and have his formula in his. Make a game of it with crackers and sips. When does your pediatrian say to transition to real milk? If it is soon try mixing it to get him interested.