Jose Eduardo Estrada, 34, of Rancho Cucamonga has been behind bars in West Valley Detention Center in Rancho Cucamonga since Monday.
Estrada was being held in lieu of $750,000 bail. He offered his plea in West Valley Superior Court.
San Bernardino County sheriff's investigators said Estrada met the boy in an Internet chat room and got his cellphone number. They began exchanging text messages and arranged to meet in person.
Estrada brought the boy to his apartment in the 7900 block of Elm Avenue starting in late October and engaged in sex acts with him, officials said.
Deputies arrested Estrada on Monday.
One of the questions asked in the booking procedure is what the inmate does for employment.
"Estrada claimed he was a major in the United States Army when he was booked into West Valley Detention Center," sheriff's spokeswoman Cindy Bachman said.
A records check with the Army indicates otherwise.
Estrada is a staff sergeant in the Army Reserve assigned to Company A, 426th Civil Affairs Battalion in Upland, according to Army records.
Civil Affairs Battalion officials declined to speak about Estrada and referred inquiries to Maj. Annmarie Daneker, an Army spokeswoman.
"At this point, it's a waiting game to see what happens to Estrada," Daneker said.
"But I can tell you that he's absolutely not a major and he's never been to officer candidate school."
Daneker confirmed that Estrada is a reservist on active duty.
Daneker said she was unsure if the Army would file any charges against Estrada.
"That's for someone else to determine after the civilian authorities conclude their case against Staff Sgt. Estrada," Daneker said.
"We don't want to jeopardize their case against him."
Detectives are asking anyone with information about Estrada to call 909-477-2800.