HELP unusual scab on rabbits head

I have just noticed an unusual scaly looking scab on the top of one of my bunny's heads. There is also a patch of hair missing from the scab. Other than the scab there is no visible damage or any unusual signs. He is eating and drinking well as well as going to the bathroom regularly. The scab is red and very bumpy. Please let me know if you have any ideas of what it could be or what I can do to help him. NOTE* the black/brown spot near the scab is just a birthmark/skin spot

Fur mites. or some other mites perhaps...
http://www.rabbit.org/journal/4-9/skin.html

possibly mites... any other rabbits? Could be an old, healing injury.