"He was like, 'Well, I didn't know what to do,' " she told ABC's Good Morning America. "'You just can't tell Jerry no.' "
The boy, referred to as "Victim 1" in court papers, met Sandusky in 2005, when he was 11, through the charity the coach founded for at-risk boys. According to court papers, the boy often stayed at Sandusky's home, where he was allegedly repeatedly sexually abused.
His mother says her son even started misbehaving so that he would be grounded and not allowed to go over to the coach's house.
She eventually shared her concerns with her son's high school, and the investigation that ensued quickly revealed that her son was only one of several boys whom Sandusky allegedly abused. The grand jury report lists eight victims, but Victim 1's mother says she believes more will come forward.
"We expected it would get big," she said, "but we never expected it would be this bad, and so many people covering it up."
She also says she only became aware of the full extent of the abuse once Sandusky was formally charged.
"I was absolutely horrified," she told ABC. "I didn't know it was that bad. It's caused a lot of nightmares – for him and I both."
The mother told ABC her son is relieved that Sandusky has been charged and hopes to prevent other children from being abused. She also says she supports the Penn State Board of Trustees' decision to fire head football coach Joe Paterno.
"I think if he had any inclination of this, he may have done what he legally needed to do, but there's got to be some moral bearing, in my opinion," she said.
As for Sandusky, she says, "I want justice. I want him to be locked up. There's no help for someone who does this. Not like this. He needs to be put away. He needs to be put away for a long time."