Considering that he didn't know they weren't his until they were 12,14, and 16 according to the article, he is probably listed on the Birth Certificate as their father. In which case, that is legal paperwork that makes him the father.
It's interesting that some of you who are normally in the "Adoption not Abortion" side are so dismissive of any definition of fatherhood that doesn't begin with sperm. As an adoptive father, I take extreme umbrage with this attitude.
You, my brothers and sisters, were called to be free. But do not use your freedom to indulge the flesh; rather, serve one another humbly in love.For the entire law is fulfilled in keeping this one command: “Love your neighbor as yourself.”