Being a Father myself, I have an emotional attachment to my children. I am lucky in that I know I am their genetic donor and their father. Sperm donor does not make you a father. Being there for your kids to the best of your ability is what makes you a father. Unfortunately biological rights usually supercede custodial rights. If the world were truly interested in whats right for the children, the guy if still interested should get custody of the children in said case on the grounds (if no problems in his past) that he is the more stable and mature guardian. In the real world he would probably only legally in a court battle be able to get custody of his biological child. The wife or the biological father of the other child would be able to get custody of that said child. What a sick world, when the welfare of the child is not truly considered. I would admit that in a court case, the judge is severly hampered by short time exposure to the participants, but they also have to uphold existing law.
There is a statement that is corny but it is accurate, Any man can be a Father but it takes someone special to be a Dad.