Hi thanks for your question.
At the outset I presume that vitals(pulse ,blood pressure) of your son are normal.
He has met an accident 6 hours back and has swelling at the back of his head.
I hope he has not lost his consciousness and has not vomitted.
He has not developed any complication other than haematoma
for last 6 hours,hope fully there will be internal bleeding
,but he need urgent CTscan head to rule out bleeding inside his brain.
Get his urgent CT head done and consult a general surgeon
for management of haematoma (swelling at back of head).
Hope this answers your question