Attorney Chuck Schmidt told The (Harrisburg, Pa.) Patriot-News his client, now 19 years old, alleges Sandusky plied him with whiskey and sexually assaulted him in a Penn State football office in 2004 when he was 12.
Schmidt said the accuser was a camper with Sandusky's Second Mile children charity at the time and plans to file suit in Dauphin County against Penn State, the charity and Sandusky.
"He's had a difficult life," Schmidt told the newspaper. "And we have reason to believe that much of it was caused by that incident."
Authorities have charged Sandusky with molesting eight boys over an eight-year period. All eight will testify at his preliminary hearing, people close to the case said Tuesday.
That all accusers would testify is a blow to the defense, because Sandusky's lawyer, Joe Amendola, had said at least one boy denied he was abused, ABC News reported Monday.
Sandusky, charged in a 40-count indictment, is accused of molesting eight boys during a 15-year period.
The grand jury report said the identity of the boy in question was not known, but Amendola told ABC News he thought the defense knew the boy's identity and talked to him.
"The kid is ... now grown up; he's in his 20s. He's adamant that nothing sexual occurred," Amendola said last month.
Amendola said Sandusky provided the boys' name to Penn State after he was confronted with accusation.
ABC News said it was told all eight boys in the grand jury document indicated they would testify against Sandusky when the preliminary hearing begins Dec. 13 in Centre County, Pa.
The revelation about the boys' testimony comes after Sandusky discussed the 2002 allegation with The New York Times, saying he never spoke about it to Penn State's head football coach, Joe Paterno, fired last month amid the scandal. He also said he and Paterno never discussed a 1998 child molestation complaint investigated by Penn State police.
"I never talked to him about either one," Sandusky said in the Times interview. "That's all I can say. ... He's the only one who knows whether anybody ever said anything to him."
Sandusky also denied the accusations.
"These allegations are false. I didn't do those things," he said.