Weird skin patch?

Okay so my boyfriend suffers from severe cystic acne, so when we started dating I made an essential oil toner for him with eucalyptus, tea tree, Frankincense, rosemary, and witch hazel. I noticed this patch of dry, red skin on his chest. I put the mixture on it and after a while it started to go away. He's been working away for the summer, and like any man😑 he forgets to put it on himself. Now the spot is the worst I've ever seen it, but I'm curious as to what it is? Does anyone know? Thanks!