Logan Mwangi’s body was found in the River Ogmore, in Sarn, Bridgend, last July, with 56 internal injuries.
John Cole, 40, told police after his arrest he had been woken by Angharad Williamson, 31, screaming that Logan was dead, Cardiff Crown Court heard.
Mr Cole, Ms Williamson and a 14-year-old each deny Logan’s murder.
The court was read a police interview with Mr Cole at Cardiff Bay police station, in which he said: “I’m certain neither of us wanted to hurt Logan, not like this.”
He said both he and Ms Williamson had both hit Logan before and that he was “clumsy and “always jumping off stuff”.
He told police Logan had Covid and was getting “more and more wound up” and jumping off the drawers on the day before his body was found.
He said he carried Logan into the bedroom and “threw him on the bed”, but could not say how he landed.
Logan later “slipped and landed in the bath” and was still “kicking off” when Mr Cole went to sleep, he said.
He said he woke up to Ms Williamson screaming, “He’s dead, Logan’s dead”.
Mr Cole said he tried CPR, but Logan’s head flopped, while Ms Williamson was “frantic and hyperventilating”.
He added Ms Williamson had said she had heard Logan “take his last breath”.
“I wish to make it clear I don’t know what happened, if anything, after he went to sleep,” he said.
Mr Cole said: “I carried him to the river and I put him on the bank in the dark.”
He said Logan’s dinosaur top had become ripped when Ms Williamson had earlier thrown him on the bed, so he “went into the woods and threw it in the bushes”.p
He said: “I literally woke up to him dead, and it didn’t make sense.
“I love Logan – I didn’t murder him, I tried to save him.”
Mr Cole said he and Ms Williamson had both lied to the police. He has admitted a charge of perverting the course of justice, which Ms Williamson and the 14-year-old deny.
Earlier on Wednesday, the court heard that a support worker for the 14-year old had heard him saying “Where are the kids? I want to kill all the kids”.
Another support worker had heard him singing about punching children in the head, describing it as “orgasmic”, after Logan’s death.
The court was also read a transcript of a phone call between Ms Williamson and her mother, in which she discussed John Cole pleading guilty to perverting the course of justice, a charge which she and the 14-year-old deny.
She said: “How is it that he can plead guilty to perverting the course of justice?
How can he plead not guilty to murder?”
“What has he done to my son? I need to know,” she added.
A letter from Ms Williamson to her family was also read to the court, in which she denied any involvement in Logan’s death.
It read: “I’ve lost my son and my ex-partner admitted to hurting child L and moving his body.
“As my parents I hope you know I have nothing to do with child L’s death.
“I’m not the monster they are saying I am in the media.”
She added she felt “alone and scared” and told them the “truth” about Logan would come out.
All three defendants deny Logan’s murder.
The two adults are also charged with causing or allowing the death of a child, which they both deny.
The trial continues.
