The "writ of summons" filed by Mike McQueary's lawyer described it as a whistle-blower case, but the brief document was not accompanied by a full complaint that would lay out the allegations.
The filing was first reported by the Centre Daily Times on its website.
McQueary's attorney, Elliott Strokoff of Harrisburg, did not respond to a phone message at his office late Tuesday. His father, John McQueary, declined to comment on his behalf.
McQueary, then a graduate assistant, has said he complained to Paterno of seeing the boy in a locker room shower naked with Sandusky.
He testified in December that he believed Sandusky was molesting the boy and "having some type of sexual intercourse with him," but added he was not "100 percent" certain they were having intercourse because of his vantage point.
Penn State's trustees have said they fired Paterno as coach partly because of his response to the incident. Paterno reported the matter to administrators Gary Schultz and Tim Curley, which trustees have called "his minimum legal duty" and "a failure of leadership." Paterno was fired as coach in November and died in January of lung cancer.
McQueary, who was a receivers coach, was placed on paid administrative leave by the school after the three men were arrested in November. All three have pleaded not guilty.
Penn State spokesman Dave La Torre said school officials were unable to comment because they had not seen the complaint.
Pennsylvania prosecutors said Monday they now believe the alleged shower assault took place a year earlier than they first claimed.
The attorney general's office said in a court filing that investigators concluded the alleged attack took place around Feb. 9, 2001. Previously filed court documents, including a grand jury report issued before Sandusky's arrest dated it March 1, 2002.
Associated Press writer Genaro Armas in State College contributed to this report.