I have a friend that has the same problem. They called it Drool rash. Your son obviously has a sensitive skin and it's okay. It will eventually goes away on its own. But make sure it's not discomforting for your son and maybe talk to your son's doctor if you have concern. But other than that, put petroleum jelly on the rash and try to keep up patting his drools to lessen the rash on his chin. Hope this helps.