There can be some possibilities for your son’s symptoms like contact dermatitis, intertrigo, miliaria rubra, contact dermatitis, drug eruption, exfoliative dermatitis, tinea pedis, staphylococcal and herpes infections
. If this is due to staph infection
, then a culture needs to be done and then specific antibiotics prescribed. Sinus infections cannot present with staph infections. I suggest you to get it evaluated from a dermatologist.