Well frontline doesnt cause this and unless you used a flea shampoo, which I know you didnt, that doesnt make any sense either as far as a reaction. I am lost as well.. let me know what the vet says.. hmmm..
If he was mostly a outside dog, coming in with a woodstove ( I know because I have one) I put water on the stove in a woodstove cast iron pot.. Maybe the excessive licking just aggrevated it..