In answer to your original question, OP, you cannot dispute paternity in any sort of legal action at this time.
Per WI law, which is what I believe rules since all parties live there and that's where the divorce was granted:
767.803  Determination of marital children. If the father and mother of a nonmarital child enter into a lawful marriage or a marriage which appears and they believe is lawful, except where the parental rights of the mother were terminated before either of these circumstances, the child becomes a marital child, is entitled to a change in birth certificate under s. 69.15 (3) (b), and shall enjoy all of the rights and privileges of a marital child as if he or she had been born during the marriage of the parents. This section applies to all cases before, on, or after its effective date, but no estate already vested shall be divested by this section and ss. 765.05 to 765.24 and 852.05. The children of all marriages declared void under the law are nevertheless marital children.
History: 1979 c. 32 ss. 48, 92 (2); Stats. 1979 s. 765.25; 1979 c. 352; Stats. 1979 s. 767.60; 1981 c. 314 s. 146; 1983 a. 447; 1985 a. 315; 2005 a. 443 s. 229; Stats. 2005 s. 767.803.
Because this child became a "marital child" upon your marriage to mother, the following applies:
A paternity action may not be used to challenge paternity previously decided in a divorce action. That paternity was not challenged in the divorce is irrelevant if it could have been litigated. Paternity of Nathan T. 174 Wis. 2d 352, 497 N.W.2d 740 (Ct. App. 1993).
So, the child is yours. You would be better served to figure out how to either modify/dispute the support amount, or figure out how to pay it, rather than trying to figure out how to *******ize the child that you've supposedly cared for as if he were your own for years.