Known only as Victim 2, his identity has remained a mystery throughout Sandusky's sex abuse trial, even to prosecutors. He is being represented by four attorneys. His name is not being released to the public.
The man's lawyers said Thursday they have done an extensive investigation and gathered "overwhelming evidence" on details of the abuse by Sandusky, who was convicted last month of using his position at Penn State and as head of a youth charity to molest boys over a period of 15 years.
The attorneys have posted on their website two voicemails that Sandusky allegedly left on the man's cellphone in 2011. In the messages, Sandusky says "Love you" and says he wants to express his feelings "up front."
Testimony about the shower incident came from assistant coach Mike McQueary, then a graduate assistant, who said he witnessed it. He said he saw Sandusky and a boy, both naked, in a team shower after hearing skin-on-skin slapping sounds.
"I would have described that it was extremely sexual and I thought that some kind of intercourse was going on," McQueary said.
McQueary reported the incident to school officials, including longtime coach Joe Paterno, but none of them told police.
The lawyers for the man claiming to be Victim 2 say he will sue Penn State for the "egregious and reckless" conduct that facilitated the abuse.
"Our client has to live the rest of his life not only dealing with the effects of Sandusky's childhood sexual abuse, but also with the knowledge that many powerful adults, including those at the highest levels of Penn State, put their own interests and the interests of a child predator above their legal obligations to protect him," the lawyers said in a news release.
The university said it was taking the case seriously but would not comment on pending litigation.
In a recent report conducted by former FBI Director Louis Freeh and commissioned by Penn State, the investigators excoriated Paterno and the other administrators for not attempting to identify Victim 2, saying it showed "a striking lack of empathy."
The Associated Press contributed to this report.