Dribble rash or food reaction? I'm thinking the former as it largely covers the area his dummy sits, he's teething so is drool central but he's never had it before and its worst on his chin where his dummy doesn't make contact.
He has had egg today for the first time which is the what makes me question whether it's a food reaction. It's not his first tooth coming through and he's never had a dribble rash before. It also came up pretty suddenly, about 3-4 hours after eating the egg.