Smith, who was last seen wearing blue basketball shorts shorts, is described as having blonde hair and blue eyes. He is reportedly 4-foot-8 and 76 pounds.
Police said that warm temperatures would continue into the week.
"It's going to be in the hundred-degree range today, and we need to get the boy found," Lt. Charlie Wilhite, of the Riverside County Sheriff's Department, told NBC Los Angeles.
"Those are very rugged foothills up there. There are a lot of rocks and areas not accesible by vehicle. There are a lot of areas where you might not see somebody unless you were right on top of them," he added.