You can't stop him from SEEKING parental rights, however, Pennsylvania family law TENDS to support the concept of preservation of the family unit when there is an INTACT marriage, regardless of paternity or DNA results, UNLESS there was some fraud or misrepresentation with respect to true paternity of the child. In your case, your husband has known that he's not the child's biological father for several years, yet has supported the child, and held out that child as his own. If your ex consults with an attorney, he'll likely be told that even with 100% DNA proof of being the father, the courts aren't likely to alter the status quo. If he chooses not to consult an attorney and bring suit on his own, he'll find that out the hard way. So yes, he can ask the court to look at the issue, but so long as the marriage is intact with no element of fraud, PA law leans towards the right of the intact marriage above all else, under the concept that the best interests of children are served by remaining within such.
If you happen to be in western PA, I can recommend Attorney Lisa Vari at 855-PA-FAMILY as a good resource, if dad should ever bring a case before the courts.

