What Is The Treatment For Circle Whitish Scab On The Head?

My 11 month old son developed a circle whiteish scab on his head. He also has a diaper rash that will not go away. Im not sure if its related. He got the diaper rash ten days ago and the white scab showed up two days later. He has sensitive skin and is allergic to diaper rash cream and most diapers but i have never had any problems with his scalp. The scab is the size of a quarter and when scratched its a red rash like wound that when drys turns into a white crusty again the next day.