Even if it's proven that she's not his biologically, he's still the dad by virtue of bonding and years invested. Not sure but he may be able to stop the DNA testing if it states in their divorce papers "two children were born of this marriage, OSD, born XX/XX/XX and YSD, XX/XX/XX". If she signed it, she acknowledged that they were both his kids. She should have said something then if she were going to do it. That's just my opinion though. Not sure how that stands legally.