as per your child's symptom it sounds he is suffering from a type of skin lesion called pityriasis rosea
it is self limitimg lesion . no need to worry . if itching is more u can give medication to subside itching .
Christmas tree rash
(pityriasis rosea) is a fine, itchy, scaly rash that usually appears first as a single patch on the chest, abdomen or back and extremities. . After this first appearance (herald patch), the rash may spread as small patches to other parts of the back and chest and to the limbs
. The rash may form a pattern on the back that resembles the outline of a Christmas tree.
Christmas tree rash usually resolves without treatment within six to eight weeks, but it can last several months. Medicated lotions may lessen itchiness and may help hasten its disappearance, but they're often not required.