Hello dear, The knot formation in head after injury is due to accumulation of fluid & blood and it is called Hematoma. The blood cells accumulated are eventually broken down and absorbed in the body. But this is a slow process & takes times to disappear completely. Till that time, symptomatic relief is obtained by analgesics preparations, ice compression & adequate rest. If it still persists or there are any other associated symptoms like fever, vomiting, loss of consciousness, convulsions, etc...then it will be better to consult his Pediatrician. Under such circumstances, a proper clinical+ radiological evaluation will be needed to rule out any pathological cause for symptoms. Till then, maintain adequate hydration & nutrition status. There is no need to worry, he will be fine. Wishing your son a good health. Take care.