Suggest Remedy For Ear Swelling

My son had swelling that formed behind the top of his left ear last Thursday. He didnt have any sick symptoms and it only hurt when we pushed on it. We took him to the ER on Saturday and they admitted him. After CT Scan and blood test results all coming back negative, by Monday the ENT suggested to make a small incision and remove whatever was in there. The procedure was about 30 mins and he then told us it was blood and should now go down. By Tuesday after they took off the bandage it seemed the blood had filled back in but they said it was something that happened and he was still discharged and told to keep the pressure on it with the head wrap. We are now at home after a day and it seems more blood is filling in so we are taking him back to dr tomorrow. We don't know the full source of the clotting or why it swelled from the start. We think he may have bumped his head at school on Wednesday or Thursday, but still the area he said he bumped it was not where the swelling and blood is. The dr seem to think everything is still ok, because all of the test are negative and even the test from the blook they pulled is negative, but I am now pretty worried because its not going away and there is no answer. Please HELP!