scalp pain

My seven year old son has been complaining of a sore scalp on the top of his left crown area.  I examined the scalp very carefully and can find nothing.  It looks healthy and normal.  He doesn't complain about pain, ever, so since he has had outbursts of "Ouch!!" for barely touching it, I feel there really is something bothering him and hurting him.  This has been going on for about two weeks.  He is a very healthy boy and has no medical problems, he isn't on any medication.  He is normal height and weight.  Any ideas where we should start? Do we wait to see if it persists?
Concerned Mom.