Do you mean mother's milk, or formula, or cow's milk? If mother's milk and he was before, see his pediatrician. If formula and you are not breast feeding, see his pediatrician. If he is not interested in drinking cow's milk but he will drink breast milk or formula, don't worry. It's too early for him to drink cow's milk anyway. But if you mean he is not eating anything at all, go right now to the doctor with him.