D.W.
14 month olds really don't require a ton of food to keep them going. If he's drinking his milk and eating yogurt he's getting fats, vitamins and calories. If he's eating veggies he's getting in some fiber and more vitamins.
Kids are amazingly resilient. Did you ask your doc how long he/she thinks this will take to run it's course? Look at that time frame then you can make decisions about how to proceed with his care if he's still not acting like his normal self after that.