If a child's diet consists of mostly milk, what nutrient is missing?

Many. After age 2 years you do not need more than 2 to 3 cups of milk per day. Milk has no iron sources so you can become anemic. If you are not eating all food groups you are missing vitamins and micro nutrients. Kids need at least 3 to 5 servings a day fruits and veggies, plus grains and legumes. To mention some things. Talk to your doc, find the best way to expand his diet and wean off the milk.