This sounds like a frustrating dilemma. While hungry children will generally eat eventually, it is very important to keep young children hydrated. That your son has developed a preference for a particular style of pouch is probably less concerning from a nutrition standpoint than the fact that he is only drinking juice. You are probably also tired of having to stock up on expensive juice pouches! To tackle the first concern, perhaps you can water down the juice, gradually increasing the proportion of water over time. Once he is drinking more water, try out one of the reusable drink pouches on the market. When you do try a cup, if he discards the beverage or throws a tantrum, continue to calmly hand him the cup every few minutes. Eventually, when he sees you are consistent and he is not getting a big reaction, he may just try the cup. Make sure that he still drinks enough during the day so he does not dehydrate. Either way, preschool children often develop preferences that do eventually disappear. This may be something you just have to wait out. Make sure to ask your pediatrician about nutrition and health concerns.