There is actually no need of doing unwanted investigations for a 14 year old child . Only if it's clinically indicated you need to do. Anyway since you have already done it I can explain about that. According to what you have said vitamin D is very less. The best source of vitamin D is Sunlight . Now a days children spend most of the side in the house or flat and are more interested in Indoor games / video games etc..Lack of sunlight is the major cause for Vitamin D deficiency. Best  thing you can do is promote outdoor games mostly in the morning or evening . Vitamin D supplements are also available that should be taken after consulting a doctor .

Signs of disease appear first followed by symptoms which patient complaints. As far as question is concerned,it's not wrong to test a child. And second, vit D3 levels needed for bone growth, as it is reduced means,his body is using excess vit to meet growth needs..so it is wise to supplement the same to have better growth