SAN DIEGO -- A man pleaded guilty Thursday to sexually assaulting a 13-year-old boy after plying him with alcohol on a secluded Poway hillside near a middle school.

Jesus Mora Nava, a 30-year-old illegal immigrant, agreed to enter into a plea deal that carries a 16-year prison sentence on the day his preliminary hearing was to take place.

The defendant admitted to two counts of committing a forcible lewd act on a child under 14 and one count of using false documents to conceal his true citizenship status.

Nava -- who will be sentenced Jan. 3 by Superior Court Judge Frank Brown -- also admitted to allegations that he was a stranger to the boy, that the child was fearful, and that he engaged in substantial sexual contact while sodomizing the child.

The defendant will have to register as a sex offender and will be deported when he finishes serving his sentence, said Deputy District Attorney Dino Paraskevopoulos.

"(This was) a very difficult case -- a very difficult case for the victim," the prosecutor said outside court, noting that the plea deal means the victim will not have to testify.

Paraskevopoulos said the teen and his family were happy to hear that Nava admitted his guilt.

A man who came upon the victim when he emerged from the hillside area after being attacked told reporters that he came to court Thursday morning -- prepared to testify -- because he persuaded the boy to tell authorities what happened when all he wanted to do was go home.

"I'm just glad it turned out the way it did," Paul Sweeney, a lead custodian for the Poway Unified School District, said outside court.

Sweeney said he had driven his truck to the area to go for a run when the boy approached him, saying he had been raped by a man.

"He had told me the man had threatened his life," Sweeney told reporters.

The judge told Nava that he avoided a life sentence by admitting his guilt.

"I think this is substantial justice," Brown said.

Paraskevopoulos said the boy was standing outside Meadowbrook Middle School on July 16 when he was approached by a stranger, who turned out to be Nava.

The defendant asked the boy in Spanish if he had any drugs, and the teen said no, according to the prosecutor.

Nava left, but came back and offered the boy some beer, which he declined, Paraskevopoulos said. The defendant left again and returned with beer, and the boy had some to drink at that point, the prosecutor said.

Paraskevopoulos said Nava then took the boy with him to a nearby liquor store, where he bought more beer and hard liquor.

The defendant then took the victim to a transient encampment near the school and gave him watermelon and more beer before sodomizing the child after he passed out, the prosecutor said.

When the teen came to and started walking away, Nava followed and threatened the boy, telling him that if he said anything about the sexual assault, his family would be killed, Paraskevopoulos said.

Using a helicopter, deputies found the man about six hours later at his encampment.