Welcome to HCM.
Since his symptoms are persisting, he will need a repeat course of antibiotics. Chest infection
if left alone could be more dangerous than the risk of repeating a course of antibiotics if indicated.
Your consulting doctor must have detected some chest findings on auscultation, warranting a repeat of antibiotic course.
Don't worry. Ensure that the swimming pool is kept hygienically clean.
Wishing your son all the best.