Peak expiratory flow is a measure of how quickly a person can move air out of their lungs. A peak flow rate will be higher when a person's lungs are functioning normally and will drop if there is constriction in the airways, as classically occurs with asthma
If you son is having his peak flow measured, it sounds like his physician may be concerned that he has asthma. There are several advantages to measuring peak flow, and it can be used to help adjust asthma medications, including inhalers. Peak flow is typically measured using a small handheld device that patients can keep with them at home. Although peak flow is not used to diagnose asthma, it can be used to help monitor it. Asthmatic patients are encouraged to check their peak flow every day--after checking for a few days while feeling well, patients get a sense of what their normal range is. Regular monitoring of peak flow can help identify when an asthma flare may be starting. In addition, if a patient with asthma starts feeling short of breath, the degree to which the peak flow drops can give a measure of how severe the flare is which can then help determine the level of additional care that is needed.
Your son may or may not be prescribed an inhaler after his peak flow test, but it is likely that he will be given his own peak flow meter to check at home.