This is an important question, and one you should definitely discuss with your pediatrician since kids between ages 1 and 3 need around 500 milligrams of calcium per day to build strong, healthy bones. When a child can't digest cow's milk, many pediatricians recommend giving soy milk instead, since the proteins and sugar that tend to cause the most trouble in cow's milk are different in soy varieties. You can also try giving your son some yogurt to see if he handles that any better. Many kids who have trouble digesting cow's milk are able to tolerate yogurt (it's much lower in lactose, the sugar that is hard for some kids to digest). A single serving can contain as much as 400 milligrams of calcium, so it's a great alternative to drinking milk.
If your child can't tolerate soy milk or yogurt, he'll have to get the calcium he needs through other areas of his diet. This means you'll have to be extra diligent about offering him calcium-rich foods like fortified bread, cereals, and juice, as well as natural sources like tofu, broccoli, kale, and spinach. Although calcium supplements seem like the easiest solution, doctors don't tend to suggest them for children this young since they're usually chewable and toddlers have a hard time taking them. Although giving your child a liquid multivitamin will provide some calcium, it's not 100 percent of the daily recommended amount.