My son was hit by a car in 2009 and since then had a bruise even after several months have elapsed. It has recently begun to experience headaches and vomiting. Why is that?

Where had the hematoma, intracranial or superficial? The superficial or subcutaneous hematomas not cause any problems, they disappear after a few weeks or months. I think the symptoms you describe have nothing to do with what happened in 2009 unless there had intracranial bleeding gradually spread (?) . Headaches and vomiting could be very well signs of an increase in intracranial pressure or, in general, something less serius.