An Amber Alert was issued for Devin Davis on Tuesday.
Liberty County sheriff's deputies said Devin wandered off while his mother and sibling were napping at the family's home about 3:30 p.m.
The area has thick underbrush and some bodies of water.
Detectives said they fear that Devin was kidnapped after he wandered off.
Devin is 30 inches tall and weighs 40 pounds with red hair and blue eyes.
He was last seen wearing a red and gray jersey T-shirt, jeans and Spiderman tennis shoes.
More than 100 officials and volunteers from all over Liberty County and the surrounding areas were searching by air and land, and said they would keep searching for the boy around the clock.
"Both parents are here, devastated and in a word -- very frantic. We're doing everything we can to find him right now," said Capt. Rex Evans with the Liberty County Sheriff's Office.
The Davis family has only lived in their home for six days and had recently moved to the area from another state.