What Causes Mottled Skin Over The Sternum?

My son has mottled skin directly over his sternum. It does not itch. His chest does not hurt. He just completed his wrestling season and about three weeks ago another wrestler s head hit him hard in this area. He complained of slight discomfort for a day or two and then the discomfort resolved. Could this mottled skin just be a bruise that is slowly dissipating? There is no swelling, at least not now.