8 Y/O son has lump on sternum...Inputs please

My 8Y/O son has been to the doctor recently because he complained about pain on his chest. The doctor noticed a little lump/bump with redness right on the sternum area. It was really painful for a day or so. The pain has subsided, and the redness gone away. They performed Xrays, and blood tests, and ruled out a blood infection, and fracture. They think its probably just a sprain or hematoma. Its been about 10 days, the lump is still there, it is not moveable. It feels as if it might be going away. The redness is gone too! Wheeww! My son is a very active kid, he indicated he might have been kicked in the chest by his sister. He is pretty rough. My question is this; if there was an abnormal growth (tumor, cancer, etc) would the Xray and blood test have picked it up? And how long if it is a sprain do these dang lumps go away? Im just happy that the doctor said everything looked okay, but he has told us to come back if hes still in pain in a few weeks. What do you think this could be? Three questions, Im a worried father over here. I apologize for the long story everyone, I appreciate all your answers! Thank you, take care