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From your description of the lesion , it looks like a swollen and tender post auricular lymph node.
Now coming to the causes, many are possible like infections (bacterial, viral etc) either systemic or scalp
. Seborrheic dermatitis
) or pediculosis
) can also cause this condition.
It is important how your child is doing clinically. It is ok to wait and watch if there is no fever and cough . You can wait as they regress on their own by few weeks.
But if the lesion continues to persist or there is increase in the size of the lymph node along with development of fever or any other symptoms, then you might need to consult a paediatrician to look for any signs of systemic infection
Hope it helps
Wishing your son speedy recovery
Dr Geetika Paul