My son was hit by a car in 2009 and had a hematoma for several months after. Recently he started getting headaches & throwing up. Why? He will get an

Get him evaluated. Where was the hematoma, intracraneally or superficially? Superficial or subcutaneous hematomas do not cause problems, they disappear within few weeks or months. I think that symptoms now have nothing to do with what happened in 2009 unless he had intracraneally bleeding that grew slowly (?) headache and vomiting may well be signs of intracraneally pressure or more commonly something less serious.
Hard to connect. More information is needed to answer your question. Was the hematoma due to a head injury? A head injury patient may suffer from headache, dizziness and even personality problems. These changes usually occur shortly after the injury and resolve within a year. While unlikely that the remote injury is the cause of the current symptoms, your doctor should know the full history of the accident.
Repeat images. Post-traumatic hematomas may recur, cause head-aches, or e3ven seizures. Head-aches and throwing up raises the concern for recurrent hematoma. It would be prudent to obtain repeat images(ideally from the same location so they have direct comparison to his past images). Head ct or MRI would be appropriate. If pathology is present, neurosurgical evaluation would be correct. Otherwise, neurology.
Intracranial pressur. You are worried about possible elevation of intracranial pressure following remote head trauma with hematoma. This could be the cause of the headaches and nausea. However, there are many common causes of headache with nausea and vomiting. A detailed history, examination and testing is the only way to determine the cause of your son's symptoms with any accuracy.