I don't have a kid of my own by any means, but Giga is right.
Just because a man doesn't have a biological link to a child in no way means he cannot be the child's father figure. Your ex may have got you knocked up, but at what point was sex difficult or stressful? This new guy is filling dirtbag's shoes and its not even legally his child. If this guy wants to act like a father and you let him, then he is a better man than I can ever hope to be.