It depends on how those 30 minutes of breastfeeding are spread over the course of the day. Is it six five-minute nursings or one 30-minute stretch? Most toddlers can get a tremendous amount of milk in five minutes, so he may be getting all the calcium he needs from you if he's nursing often. The nutrients in your milk will meet his nutritional needs much more completely than the milk of another species (i.e., a cow). It also depends on the other foods he's eating. Cheese, yogurt, leafy greens, and fortified juices are good sources of calcium too.