He thought they were his friends.

The 16-year-old autistic boy allegedly assaulted by two teenage girls in Southern Maryland is perplexed by the criminal charges they are facing and even considers one of them his girlfriend, according to the boy’s mother.

“He doesn’t appear to be traumatized. He thinks these girls are his friends and is surprised the police are involved,” said his mother, who works for a local health department. “But I am glad they are [investigating]. I am glad someone brought this out.”

The Post is not naming the mother to protect the identity of the victim, who is a minor.

News of the criminal charges in mostly rural St. Mary’s County has rippled across the Internet, creating a furor. Police said the two girls — ages 17 and 15 — assaulted the boy repeatedly between December and February and used their cellphones to record the attacks. The videos allegedly show them holding a knife to the victim’s throat, forcing him to perform various sexual acts, kicking him in the groin and dragging him around by his hair.

Lauren Bush, 17, of Mechanicsville (Courtesy of St. Mary's County Sheriff's Office)

At least once, the two suspects lured the victim onto a partially frozen pond to chase a ball even though they knew the ice was thin, police said. The boy fell through the ice several times, but the suspects refused to help him out of the frigid water, police said. The boy was able to pull himself out of the pond, according to Sgt. Cara Grumbles, a spokeswoman for the St. Mary’s County Sheriff’s Office.

The 17-year-old, Lauren A. Bush of Mechanicsville, is charged as an adult with two counts of first-degree assault, two counts of second-degree assault and other charges. She is being held at the St. Mary’s County Detention Center.

The 15-year-old, who is charged as a juvenile, also is facing assault counts, authorities said. She is being held at the Waxter Children’s Center, they said.

Police said the investigation is ongoing.

Voice mails left by The Post for Bush’s parents produced no response.

All three teens attended Chopticon High School in Morganza.

In her interview with The Post, the mother said she has pressed her son for details about what happened between him and the girls. But her son, who was diagnosed with autism in elementary school, hasn’t been forthcoming and shrugged off the alleged assaults as playful behavior. The mother said her son is fairly independent and performs well in classes but is socially “naive” and cannot comprehend that he may have been manipulated.

“He’s not a good judge of people,” the mother said. “I keep trying to talk to him about it, but it’s hard to get much out of him. I’ll say, ‘This is what so-and-so told me, and I know this happened,’ but he’ll act like, ‘Oh, [the girls] were just playing around.’ He didn’t deny anything. I am trying to make him understand that people are talking about this all over TV, but he wanted to go to school today.”

The mother said she learned Friday that police were investigating the incidents involving her son but didn’t learn the details until this week. “Apparently, these girls recorded all this stuff on their phones, and someone saw it and reported it,” she said.

The boy and the two girls socialized frequently, the mother said, and she had heard their names but had never met them. The girls would go to movies with her son and hang out at the boy’s home on snow days. The mother said she believes that the attacks against her son took place when she and her husband were at work on days when snow closed schools.

She knew about the incident involving the partially frozen pond, she said. When she got home from work that day and asked her son why his clothes were wet, he said he had tried walking on the pond and fell in. He denied that his friends coerced him, she said.

Wednesday evening, the mother said she had not seen the videos of her son being assaulted and has not called the parents of the girls. Several weeks ago, she said, she considered calling the younger of the two girls — the 15-year-old who her son believes is his girlfriend — because he had been spending a lot of his savings on her.

With so much outrage pouring out online, the boy’s mother said she is trying to reserve judgment about the girls’ parents. “I can’t judge them. They don’t know what their daughters are doing, I assume,” she said. “I fear that people are going to blame me and say, ‘You have this mentally challenged kid; why don’t you know where he is all the time?’ But he has a learner’s permit, and he’s been coming alone from school since he was 12. He can take care of himself, but he trusted these girls.”

The mother said she worries that her son is not grasping the full scope of what happened. “We told him that what these girls did was unacceptable,” the mother said. “But yesterday, I asked him, ‘You’re going to stay away from these girls, right? You don’t plan on hanging out with them again, right?’ He said, ‘I don’t know.’ ”

Jennifer Jenkins contributed to this report.