Don't you have to be over 18 to be bound by anything in a court of law? Did he have a guardian ad litem when he admitted to paternity at age 15? Sounds pretty screwed up all around.
I think THAT is probably the best argument he could make..
Though, to be honest, the Courts see this as having someone responsible for the care and upbringing of a child.
Same thing with Statutory Rape. Doesn't matter if she LOOKS 18, she better BE otherwise somebody is going to jail.
These two issues (child support and underage sex) are the two moral laws the Courts enforce. Goes with the suggestion that it might be good to know the parents and perhaps even get married before you go have fun in the sack.