Hi welcome to Health care magic forum. Your son had an injury
3 weeks ago,wound is attended and healed. swelling is still prominent. most probably the persistent swelling is due to fibrosis
of the surrounding tissues, as there is no pain and it is hard it might be that.
I advise you to consult your doctor again for this purpose, he will examine the swelling and do the need full. No surgical procedure is required, it will be cleared by medicines only.
Besides i advise you some diet recommendations as avoid giving him potato, other tubers, pulses, and fish. comulsorily give him juices of citrus fruits like lemon, orange, etc.
Give him more of green leafy vegetables, milk, meat, egg, chicken, soups, fruits juices, and dry fruits. which improve blood, general health, resistance and stamina to with stand the pros and cons of the school life. Wishing for a quick recovery and thanks for calling.