Arson investigators did not identify the boy or provide details about where he allegedly started the blaze.
The news comes as firefighters are finally bringing blazes across the Southland under control.
Forecasters are predicting offshore winds by the weekend, though the ferocity was expected to be less than the gale-force winds that spread wildfires through the region's bone-dry tinder last week. Moist air and lower temperatures in recent days have helped firefighters battle the blazes.
"We've had good weather for firefighting," said Rich Phelps, a U.S. Forest Service spokesman. "This is a window of opportunity we have to take."
Of the roughly three dozen fires that have burned more than half a million acres, seven were still not contained. Four fires were nearly under control: In San Diego County, the Witch fire was 95% contained; in San Bernardino County, the Slide fire 90% and Grass Valley fire 95%; and in Los Angeles County, the Ranch fire 97%.
Still burning in San Diego County was the Harris fire at 75% contained and the Poomacha fire at 65%. In Orange County, the Santiago fire was 65% contained, with full control expected Friday.
"The last person's not coming off the line until the perimeter's cold," said Battalion Chief Kris Concepcion of the Orange County Fire Authority.
The work will not be over once the fires are extinguished. Recovery plans designed to prevent mudslides on the now-barren hillsides during winter rains will need to be implemented.
The fires destroyed 2,786 structures in Southern California, including more than 2,000 homes, according to the state Office of Emergency Services. Seven people have died, and about 500,000 were ordered to evacuate their homes. Most evacuation orders have been lifted across seven counties in Southern California.
Overjoyed residents of Orange County's rural canyons returned home Monday for the first time in a week.
"Oh, my God, I'm so glad to be home," said Tim Stinson, 56, half laughing and half sobbing as he hugged a neighbor from Modjeska Canyon, where several homes burned to the ground. "I don't think I've cried in 50 years, but I'm crying today."
But some mandatory evacuations remained in force, including Silverado Canyon in Orange County, where 200 homes were still threatened. The fire was burning slowly down the south side of the back of the canyon. Black Star and Baker canyons also remained under mandatory evacuation.
The Santiago fire was burning northeast through heavy chaparral in the Cleveland National Forest, and firefighters had six miles of fire line left to build before they planned to pinch off the blaze above the Silverado Canyon area near Modjeska Peak, Phelps said.
The Orange County blaze, which has scorched 28,400 acres and destroyed 15 homes, was believed to have been started by an arsonist Oct. 21 along Santiago Canyon Road. A $285,000 reward is being offered to anyone who offers information that leads to a conviction.
Authorities were seeking the driver of a white Ford F-150 pickup truck seen in the area when the blaze began and were hoping to speak with 35 to 40 people who had gathered near Black Star Canyon Road and taken pictures and video of the fire in its early stage. "They are not suspects. We just want to talk to them" and look at any photos or videotape they have, said Orange County Fire Authority Chief Chip Prather.
Volunteers are trying to reunite with their owners dozens of horses that were hastily evacuated from the canyons. Photographs of some of the horses, which are being tended at the Coto De Caza Equestrian Center, can be viewed at www.cacares.stirsite.com.
The nearly 13,000-acre Slide fire was still threatening about 2,500 homes in Running Springs, Smiley Park and Green Valley Lake, officials said. Nearly 2,000 firefighters were trying to extinguish dead trees still burning in the heavy timber around the fire's perimeter.
In San Diego, Mayor Jerry Sanders told the City Council that he had ordered a study of the city's response to the fires that destroyed an estimated 365 homes within the city limits.
Sanders praised firefighters, police and innumerable volunteers and city employees, concluding that their response "proved we are truly America's Finest City," the city's preferred nickname. At the peak of evacuation efforts, San Diego's shelters housed 27,086 people at 50 public and private shelters, officials said.
On Monday, the Internal Revenue Service announced that residents affected by the wildfires would have until Jan. 31 to file federal withholding tax returns and estimated tax payment for the fourth quarter.
Times staff writers Michael Muskal, Tony Perry, Maeve Reston and Francisco Vara-Orta contributed to this report.