There is no evidence that he had any symptoms on the way to the US. Also he was still in denial last Sunday when his stepdaughter forced him to go back to the hospital. Speaking of her, she was not in denial, she made the paramedics go back to the ambulance to get their gloves and masks. After Duncan was at the hospital in isolation she came back and disinfected the apartment. Probably saved the lives of the three others there (Troh, the GF is toast IMO). She also risked her own life taking the victims blood pressure and temperature a couple times. But she was also careful to wash her hands. If she survives she will be a lesson of how to deal with this if you don't have a hazmat suit.
There are unconfirmed reports that he puked on the plane.
The government denies this, of course, but the person who was sitting next to him reported it.