hello you seem to have described what could be a nevus(mole) or birthmark. It is possible to be present for many years. You have not mentioned the age of the child or his normal skin colour. But it seems to be what is called a halo nevus. They are normally harmless and don t require treatment. Do keep a watch on it for progressive increase in size or appearance of new moles elsewhere. Look for any other white patches or large moles/discolouration on the body. Since it is hidden by his hair in this case, it can be left alone. In case it is enlarging or you wish to have it removed, procedures such as cautery excision of the dark area has been shown to clear the the discolouration. But again keep in mind his age and observe for now.