I'm not the person you were asking, but I agree that keeping this a secret seems like a mistake. Between the increasing popularity of genetics & ancestry services and the increase in health & fitness tracking, it seems extremely likely that your child would eventually find out that they aren't related to your side of the family and/or have half siblings out there.

The lie seems potentially much more damaging to your relationship than the fact that you are a loving father to a deeply wanted child who does not share your blood.