A Riddle

I saw this question on a forum and decided to post here:

A young boy and his father were out playing football when they were caught at the bottom of a giant pileup. Both were injured and rushed to the hospital. They were wheeled into separate operating rooms and two doctors prepped up to work on them, one doctor for each patient. The doctor operating on the father got started right away, but the doctor assigned to the young boy stared at him in surprise. "I can´t operate on him!" the doctor exclaimed to the staff. "That child is my son!"

Why is that so? Quick, come and give your answer in the comments!