Talk to Dermatologist on Allergy

my son is 4 months old. From past 13 days , He is having allergy on his face. It started with just a spot and spread on his belly back and underarms. His face is most effected with this allergy.From yesterday,it is turning black. I am giving him kestine on the recommendation of a doc. Kindly guess de what it is and how much time will it take to heal. Attaching picture.