Raymond Southall, of West Bromwich, West Midlands, disappeared shortly after fishing with his father Dean, 31, at the Billing Aquadrome leisure park, Northampton, on Thursday afternoon.
More than 100 police officers searched the 235-acre park throughout Thursday night and yesterday.
Raymond's mother, Sharon Southall, 31, and his grandfather, also called Raymond Southall, 52, appealed for help in tracing the boy.
The holiday park, which houses about 1,000 caravans, is built on the site of old gravel pits and contains about 12 leisure lakes or ponds. It is bordered by the River Nene.
Chief Superintendent Bob Thorogood, who is leading the investigation, said the area was a "nightmare" to search. He said police had unearthed no positive sightings of Raymond since early yesterday afternoon.
Mr Southall said of his grandson: "He would not let anyone take him. He has been brought up not to talk to strangers."