i am a paramedic and have dealt with something like that. it turned out that the patient had changed deodorant and was allergic to a chemical in it. if your son has swapped deodorants it is likely the case so have him stop use and take some benedryl for a couple days and should clear up.
my adult son has an inflamed rash under his armpits that has blistered. it even hurts to go in the shower. what is it? . has had it about a week.