There are plenty of sources of calcium outside of dairy products. These include fruits, vegetables, beans, cereals, some types of orange juice, etc.
If your son is also eating fish, meats and eggs, then he's also getting some vitamin D from the diet which is required for the maintenance of blood calcium. Vitamin D is also formed from a byproduct in the formation of cholesterol (which your body makes naturally) in reaction to sunlight -called cholecalciferol (i.e. your son's body will make vitamin D while playing ball in the sun!). Younger children also have a lower requirement for calcium than older adults.
Hope this helps some, try not to fret the calcium too much!