While I am not receiving that question I feel your pain. My son's father still hates me for even daring to go against his demands and having my son. We have not seen him in well over a year and he never calls to ask about my son.
My son is 4 now and last summer he actually found someone to name daddy. I never really paid much attention to the daddy thing, Alex never seemed to really care. His first social contact was with kids on an university campus and most of the kids there just had mommies picking them up. Daddies never really crossed his mind. When we moved home and he started daycare at a new facility it was a whole new world. Most of the kids at this place are picked up by their dad's. I've even heard kids in his class talk about things their daddy did with them last night or over the weekend.
After a couple months of this my son simply named someone his daddy. I had only been with my boyfriend for a little over a year (he's one of those best friends of many many years turned boyfriends) but Alex flew out of the house one day right to him yelling "Daddy!" He figured out on his own what kind of a person makes a daddy. He simply looked at the situation and thought, "what makes someone a daddy, what do they do, what do they say, how do they take care of me?"
For us a daddy isn't biological. It's the male companion of mommy who loves him and helps take care of him and who will play ball in the house with him. As far as I'm concerned, anyone can be a father, but it takes more to be a Daddy.
When the time comes I will simply say that yes, this guy (a**hole) is your father but he doesn't love you in the same way your daddy does because daddies are the people mommies are with who help you and mommy out, love you and take care of you. Fathers only help create you. Anyone can be "father" but only someone special is "Daddy".