Joel Loffley, 29, has denied a charge of murdering his partner's son at their Orakei house last November and is on trial at the High Court in Auckland.
A police interview with Loffley, recorded on the day of JJ's death, was played to the court today.
Loffley told police he had never hit his kids and was not responsible for the death of his stepson.
"I never assaulted my kids. I've got a few from assaulting my kids' mums but never my kids, ever, ever my kids. Ever."
He said he was doing a "Man Alive course" with anti-violence education provider Shine and was trying to complete his probation for a charge of male assaults female.
"I was just trying to keep out of trouble, just do my all, my probation things correctly and try not to breach so I go back to prison."
Loffley told police that JJ's mother, Josephine Lawrence, went out and he had been looking after JJ and tidying the bedroom on the day the boy died.
He said JJ was jumping on the bed while Loffley "fluffed the blankets".
"When I've gone to fluff it the last time he's jumped, fallen off the bed and landed on his face, on his chin and his legs have gone up over his head."
Loffley said JJ was wheezing afterwards but he put that down to the effects of a recent throat infection.
He said he picked JJ up and the child stopped crying. He checked the child's nappy and found it was full so took him to the shower.
After the shower, he put JJ to bed for a rest and went out with his friends before returning about an hour later.
"I went upstairs but I thought I'd check on Josie first and see if she's checked on that boy. She said no, so I went and checked on him and found him like he was gargling on his back and there was s*** coming out of his mouth."
He said he took the boy to his mother.
"And then she screamed and then I screamed and then I started to panic and then she started to scream ..."
Earlier, Loffley told police he met JJ's mother Josephine on a social networking website.
He said she let him move in because he needed a bail address while he faced a charge of male assaults female. The charge related to an alleged assault on his ex-girlfriend.
The trial continues.
