Sului Piew was with his three-year-old sister at the time. She has told police that a man took her brother by the hand and led him away.
Residents of U Thong district have joined the hunt and divers have been searching a large pond.
Suphan Buri Governor Nimit Wanchaithanawong said yesterday there had been no sightings of the boy or other clues about what happened.
He said investigators were examining all possibilities, and had even asked the plantation owners to harvest their crop now, much earlier than usual, to help with the search.