Puzzle #5

A father and his son were driving to a game when their cars stalled on the railroad tracks. In the distance, a train whistle blew. The father tried to the start the engine, but in his panic, he couldn't turn the key, and the car was hit by the train. An ambulance sped to the scene and picked them up. The father died on the way to the hospital. The son's condition was very serious. So they rush him to the emergency room. The surgeon, on seeing the boy, said "I can't operate on this boy -- he's my son".

How could it be? The surgeon was not lying or mistaken. No time travel, adopted child, etc.

(This puzzle comes from here)