Disbelief
Eric was shocked at what the doctor told him. How could he be infertile? He had a son! Then, the doctor explained that was the reason for not wanting to say anything. This doctor knew the family before Steven was born, so this discovery had implications.
Certainty
The doctor went on to say that there was a chance Eric was fertile in the past, but it was too hard to be conclusive about such hypotheses. However, they knew with certainty that Eric was infertile around the time his son was conceived.