Lucas Dobson slipped into the River Stour in Sandwich, Kent, on Saturday afternoon, while on a fishing trip with family members.
His aunt said Lucas's father jumped into the water after the child fell between a jetty and a boat but "the current was too strong, he had already gone".
An army of public volunteers joined specialist search and rescue teams scouring river banks and waterways on Sunday morning after darkness halted rescue efforts after 10pm on Saturday.
Lucas's aunt, Maciee Stanford, 18, told the MirrorOnline news site: "He was with his dad and his dad's friends along with other children.
"The incident happened because our Lucas was on the jetty and tried to step from there on to the boat but he fell in between the jetty and the boat.
"As soon as he fell the three adults jumped in after (him) but the current was too strong, he had already gone in the short amount of time he could not be found.
"They fish and do this regularly as it is behind one of the men's houses, all the children play together here."
A Facebook page called Search For Lucas said he was wearing a black and red striped T-shirt with white shorts.
Volunteers were warned about their own safety and told by police to stay away from the water's edge, work in groups, and make sure they had phones.
Officers were first called to Richborough Road at about 1.20pm on Saturday, Kent Fire and Rescue Service (KFRS) said.
KFRS assistant director Chris Colgan said: "We're all incredibly grateful to everyone who has given everything today to try and locate this little boy."Our thoughts are with him and his family tonight at this very difficult and emotional time."