The child said in a pre-recorded interview played to jurors at the High Court that Marion Vita looked grey and was still on the sofa at the house in Baillieston, Glasgow.
He said: "She was not moving at all. She was lying on the sofa, sort of on her side."
The boy was giving evidence at the trial of Tony Vita, who has pled not guilty to murdering Crown Office manager Mrs Vita, 48, by stabbing her to death after it emerged she was having a lesbian affair.
The child had been dropped at the house by Mrs Vita's friend, Anne Tolland. He said he tried to open her eyes and talk to her before a 999 call was made.
He also described how he had discovered messages on his iPad that led to Mr Vita's discovery of Mrs Vita's affair with Elwira Rumniak.
He told the officer: "I had just got my new keyboard for my iPad, typed in a few letters and up it came." He told Mr Vita that he did not know what it was.
The boy said Miss Rumniak had been one of Mrs Vita's best friends, but that Mr Vita had suspicions about their relationship.
He added that he had previously been aware of the couple arguing.
A letter read out to the court also revealed Mr Vita claimed he had thought it was a practical joke when he was told she had died. Mr Vita, who had already been charged by police, said he did not even know she was injured after she met to speak to him following the revelation she was having the affair.
Mr Vita made the claim in a letter to a work colleague weeks after his wife's death.
In it, he also described how he had been grief-stricken at Mrs Vita's infidelity and had tried to kill himself.
The jury heard how Mr Vita had exchanged letters with Suzanne Corrigan, a colleague at the Department for Work and Pensions, following the death on September 20 last year.
Mr Vita recalled the night of the alleged killing and how he had been "overwhelmed with grief" at his wife's affair.
The trial has been told how Mrs Vita went to the family home that evening. She was later found by a friend slumped on a couch with a hole in her back.
An extract read out in court included: "I did not know she was hurt, never mind dead.
"I didn't know until I was discharged from hospital. I could not believe it and thought that it was a practical joke."
In another letter to a colleague, he said he took a cocktail of tablets over the "deceit".
Mr Vita claimed he later went to get a knife to kill himself when his wife was there, and that she became "hysterical".
He said she was sitting on the couch when he left her to go upstairs and stab himself. He added: "If I thought Marion had been injured in any way, I would have called for help."
The trial continues.