The sheriff's office says the family was camping on property they own and Nathan Hernandez was picking berries and playing hide-and-seek with his sisters when they lost sight of him at around noon. The family searched for him until dark and then called for help at about 9 p.m.
The sheriff's office says searchers have been using dogs and heat-detecting technology in the overnight search. They are also checking out buildings and vacant cabins in the area.
A footprint was found in the mud Friday morning and searchers are attempting to track it.
The missing boy is described as Hispanic, approximately 4 feet tall, with dark hair and a thin build. He was last seen wearing a black and red striped sweatshirt and jeans.
Investigators also are questioning family members who were at the campsite - a mother, uncle, two sisters and a family friend - as well as sex offenders known to be living in the area.
Anyone with information is asked to call 911.