"Every time you talked to her, it was a little bit different about what happened that day," Kyron's father, Kaine, said Thursday on The Oprah Winfrey Show, Portland TV station KGW reports.
Though he initially supported his estranged wife even after authorities focused on her, Kaine says he became suspicious the day Terri failed a polygraph test.
Kyron's mother, Desiree Young, who previously told PEOPLE she believes her son is stashed somewhere, also appeared on the talk show wearing a button with Kyron's picture.
Asked by Winfrey what the public can do to help, Young said she just wants "everybody to know my son, to know who he is and to look for him."
The interview comes three months after Kyron disappeared from his elementary school in the Portland area and a day after Multnomah County police announced they have delegated a task force of 8-10 detectives to continue the investigation.
Sheriff Dan Staton also said officials would reveal surprising info – "knowledge of things we wish we didn't know" – after the case has been solved.