The body fights infection in many ways. There are many lymph nodes in the head and neck area. When a child is sick, the doctor will often feel to see if any of these lymph nodes are enlarged or tender. However, by careful examination they may help pinpoint where the original infection began and may raise a suspicion of a bacterial cause which most often requires antibiotics as opposed to a viral cause which will go away by itself. However, the cause of the original infection which subsequently leads to cervical adenitis is most often never identified.
Bad luck is most often to blame.
Rarely, a child may have an underlying cause for being more susceptible to this type of infection. These include a cyst in the neck or a problem with the immune system, but usually there are other clues that this may be going on. Bacteria exist all around us on the skin, in the mouth, and throughout the environment. Most of the time they cause no problem, but occasionally they get in the body and cause disease. It is not surprising that your doctor could not find the reason why your son developed this infection.