Under Oregon law, O.R.S. Sec. 109.070, I don't see anything that would prevent a third party challenge to paternity where the presumption of paternity results from marriage and the spouses are no longer married and cohabitating. Oregon case law does not do a good job spelling out the factors that a court may consider before precluding such a petition or considering the best interest of the child, based upon such factors as a delay in bringing a legal action or the existing parent-child relationship. If a case is filed in Oregon and cannot be resolved in a manner favorable to the current legal father through a DNA test, it would make sense to discuss the details with an experienced Oregon family lawyer, and to discuss possible defenses (such as an estoppel defense) to any effort to initiate a paternity / filiation action at this late date.