The parents of a 15-year-old Missouri boy who was discovered in the house of a man who allegedly kidnapped him four years ago believe their child was sexually molested.

Appearing on The Oprah Winfrey show, Craig and Pam Akers nodded their heads "yes" when the talk show host asked if they believed Shawn Hornbeck, now 15, had been sexually abused by Michael Devlin, who pleaded not guilty Thursday to kidnapping charges for the abduction of another boy.

Shawn has not yet told his parents about what happened in the four years he lived in Devlin in his suburban St. Louis apartment, the Ackers' said.

Shawn said he was not ready to discuss details of his abduction and the subsequent 51 months he spent living with his accused kidnapper.

Akers said she does not know if her son was tortured, but her husband stressed that something happened to dramatically change the boy.

"There's mental torture, there's physical torture," he said. "I have no doubt that mentally he's not the same boy he was."

Winfrey said the boy told her off-camera that he was "terrified" to contact his parents during the last four years. And the boy's parents told Winfrey they have not asked their son what happened on the advice of child advocacy experts, but are convinced their son did not contact them out of fear for his life, theirs or other members of their family.

"There has to have been something held over his head," said Craig Akers, the boy's stepfather. "There's no way in the world that if he was able to do whatever he wanted to do ... there's no doubt in my mind he would have (come home)."

The boy's mother, Pam Akers, told Winfrey she believes her son will tell her what he went through.

"I believe he will talk to us eventually," she said.

Shawn said prayed for a reunion with his family.

"I prayed that one day my parents would find me and I'd be united," he said in the pre-taped interview in his home, which aired on Winfrey's talk show Thursday.

Shawn and another boy, 13-year-old Ben Ownby, were found in Devlin's apartment days after Ownby disappeared after he stepped off a school bus.

Devlin has been charged with abducting both boys.