His groin lump could indeed be a hernia that has resulted from a congenital weakness in this area, exacerbated by coughing. It could also be an enlarged lymph node, caused by his viral infection.
I would have him reassessed by his pediatrician. If your son still complains of a groin mass (either continuously or intermittently), and the pediatrician cannot find a hernia or other groin abnormality, then you might want to request a consultation with a surgeon.
Sincerely/ Robert A. Wascher, MD, FACS
http://doctorwascher.com