There are a few different causes of an elevated white blood cell count. Since the white blood cell are involved in fighting off infections in the body, almost always a white blood cell count that is elevated means that your son has just been fighting off an infection.
The most common scenario in which this occurs is that you bring your child to the pediatrician
for a regular checkup visit, and the doctor
orders routine checkup blood work that comes back with a high white blood cell count. This typically just means that your son has just gotten over some viral infection (like a common cold), and the usual thing to be done is to just repeat the blood work in a few weeks to make sure that the count has come down.
In rare cases, a high white blood cell count can be a sign of a serious problem, most typically a blood cancer like a leukemia or a lymphoma
. Even though this is not at all common, this is the reason that your pediatrician is being cautious and checking additional blood work. It will be important to followup closely with them to make sure that everything turns out normal.