Bets, that is odd. I know that if dogs get bit and it is itchy, they can chew on it and then cause dermatitis which can look like a series of small bumps and needs antibiotics, but I don't think he could have chewed the top of his head. Dogs can also have allergic reactions just like we can, so that could be the problem. There is a blood test to check for allergies, though it is expensive.
Let us know if she learns anything more.
"Dogs are not our whole life, but they make our lives whole."