Does your son drink formula or cow milk? If cow milk you may want to look at NIDO's kinder drink they make, it is designed for toddlers to young kids. Not as rich as full fat NIDO full fat milk/easier to digest.
You can find this sometimes at Super Walmarts in the dry milk section or on Amazon.
If you do use say powdered instant dry milk, flavoring it is a big thing...otherwise it lacks in flavor!