By Ian Simpson

BELLEFONTE, Pennsylvania (Reuters) - A third accuser of Jerry Sandusky told jurors on Wednesday the former Penn State assistant football coach performed oral sex on him when he was a boy, then used threats to keep him quiet about the abuse.

The 25-year-old man said he first met Sandusky in 1997 while attending a summer camp of the Second Mile, a charity founded by Sandusky, but began a relationship with him the following year after attending a second camp, when he was 11.

The witness, dressed in a purple shirt and tie, said he had gone to Sandusky's home in the fall of 1998. With no one else apparently in the house, the two went to his basement and began wrestling on the floor, he said.

"He kind of pinned me to the ground and pulled my pants down and started performing oral sex on me. I freaked out," he said, his voice low.

The accuser's account came on the third day of testimony in the high-profile case, which has shaken Pennsylvania State University and its long-successful football program, while refocusing the spotlight on the issue of child sexual abuse.

Sandusky, 68, known prior to the accusations as a top-flight football coach who dedicated himself to his charity, today faces 52 counts of abusing 10 boys over a 15-year period. If convicted on all counts, he faces a sentence of more than 500 years in prison.

The trial is taking place amid a heavy media presence in the small town of Bellefonte, the seat of Centre County, Pennsylvania, not far from State College, where Penn State's main campus is located.

Accounts from a handful of witnesses at the trial this week - including two victims on Wednesday - have portrayed Sandusky as a predator who used his charity and his reputation to win friendship with boys, who he then molested.

The accuser who on Wednesday recounted being "pinned" to the ground by Sandusky later told jurors that the former coach coerced him to keep quiet about the incident.

"He told me that if I told anybody I would never see my family again," said the man, who was living in a foster home at the time and is known in court documents as Victim 10. "I didn't say anything. Then he apologized for saying that and said he loved me."

Another witness on Wednesday told jurors he dealt with Sandusky's advances by "putting stuff in the attic, closing a door to it."

In his case, the witness, now a 27-year-old man, told jurors that he met Sandusky at a Second Mile summer camp in 1995 when he was 11.

Sandusky offered that fall to take him to Penn State football games and he went with him to home games from 1995 to 2000. He would pick him up in his car and while driving would put his hand on the boy's knee, sometimes gripping it so hard that it hurt, the man testified.

During their relationship, the boy stayed overnight at Sandusky's home in an upstairs spare bedroom. The coach, sometimes shirtless and in mesh shorts, would come in as the boy was reading comics before switching out the light and lie next to him, putting his arms around the boy and stroking his nipples and chest.

"I would say I was ready to go to sleep and he would leave," said the man, described in court papers as Victim 7. "To this day I'm sort of repulsed by chest hair ... for whatever reason, looking at it, I just hate chest hair."

The man said he showered alone with Sandusky at Penn State athletic facilities six times. At one point the coach bearhugged him from behind in the shower while both were naked.

Earlier, John McQueary, the father of former graduate assistant coach Mike McQueary, testified that his son was "distraught" when recalled seeing Sandusky molesting a boy.

The elder McQueary said his son called him directly after witnessing an incident in a locker room shower, which Mike McQueary also described in testimony on Tuesday.

"His description to me was a slapping, rhythmic kind of thrusting, slapping sound," John McQueary said he was told by his son. That is similar to the description of the incident Mike McQueary already provided to the court.

The victim in this alleged incident has never been found.

John McQueary told his son to leave the Penn State athletic facilities immediately and the two then met at their home to discuss the incident further, he said. They agreed that Mike McQueary should report the incident.

"We determined that it was imperative to report this to the authorities at Penn State," John McQueary said. They agreed to report it to Mike McQueary's direct supervisor, head football coach Joe Paterno.

Paterno, the winningest coach in major U.S. college football history, was later fired by the board for his handling of the matter. He died of lung cancer in January.

Reuters' policy is not to identify victims of sexual crimes.

(Additional reporting by Matt Morgan; Editing by Paul Thomasch and Eric Walsh)