My Son is 3 months old and he's underweight as he doesn't like milk much. The nutritionist recommends us to give him Polycal. Actually this is just carbohydrates and it can be dissolved in cold and hot drinks. For babies, you can give him 2-10% of the milk amount, say your baby takes 100ml milk, then you may give him half spoon of Polycal (2.5g) to try for a few days. If his poh poh is normal, then you may slowly increase the amount. It's not expensive, about $40 a can. You can buy it at Watsons.