If a pregnant woman agrees, along with the father of the child, that she will abort the baby, and then chooses not to do so, the father is not exempt from providing child support for the child. The mere argument of not wanting a child or not expecting its birth is not an argument against having to support it, and there is no legally binding contract in place just because the mother stated that she would go ahead with an abortion.
The Rules for Child Support
There is no legal statute in any state that says a woman is required to go ahead with an abortion under any circumstances, including those where the father of the child either expects or wants her to do so. Interpreting the privacy rights in the Constitution, the Supreme Court determined in Roe v. Wade that the mother has the ultimate right to make this decision up until the last minute, and cannot be penalized for it. Not requiring the father to pay support because she said she would not have the child would likely be considered penalization.
An abortion is, by law, the choice of the woman carrying the baby, and regardless of what she says to the father or what he expects her to do, the fact that the baby is born means that he is responsible for paying child support, provided that he is legally the biological father of the child and has not otherwise terminated his parental rights.
The only way to avoid paying child support is to terminate any and all parental rights to the child - something you may not do unless both the child's mother and the court agree to it.
Getting Help
To get help understanding what options - if any - you may have for dealing with child support for an unwanted child, you should consider speaking with a lawyer.