Coughing is a symptom of COVID-19....(along with fever, shortness of breath etc...any sign of those???)
The passenger was coughing.
It does NOT follow from that however that "the passenger has COVID-19"
There ARE MANY causes of regular, even largely continuous coughing. Did you TALK to him or his wife? They might easily have set your mind at rest. I think there is a greater chance you will suffer harm from the stress you are placing on yourself than from the mere possibility he had something contagious, and that you got it.