Taylor said he found Ihaka lying face down.
He rolled him over to check on him and with help from the night light noticed "a little bit of blood around the edge of his mouth".
Taylor said he turned on the room's light and saw Ihaka "pale white and not really
breathing".
He ran to tell Stokes something was wrong with Ihaka and emergency services were called.
Ihaka was rushed from the couple's Truman Rd property to Christchurch Hospital where he soon died.
The Crown and the defence agree there were no intruders on that fateful night and that the boy's 59 horrific injuries, including broken bones, were not accidental.
The Crown says Taylor was suffering from sleep deprivation, headaches, and irritability caused by multiple concussions around the time Ihaka was developing an ear infection, when he allegedly caused the fatal injuries.
Taylor has blamed 21-year-old Stokes for the death.
Stokes, who was 36 weeks pregnant at the time of Ihaka's death with her second child, repeatedly denied the accusations when she gave evidence this week.
Taylor told the interviewing detective how Ihaka, who was on the verge of walking unaided, "does fall over quite a lot".
The night Taylor said he heard the bang, he said it "sounded like he'd fallen over and banged his head on the crib which he's done quite often, which he'd done the night before as well".
"I've seen him fall in his cot trying to stand up," he said, saying he'd seen him "headbutt" the cot and fallen sideways.
"I've seen that numerous times."
The detective left the room for several minutes while Taylor is seen sobbing, with his head in his hands.
When Thomson returned, he told Taylor: "Some things don't seem right here to me."
He accused Taylor of "holding a few things back" and that the boy's injuries didn't support his story.
"I would never ever lay a finger on that boy," Taylor said.
"He means the world to me. I love that kid like he's my own son. He is my son. I would
ever do anything to hurt him."
Thomson then accused him of hitting Ihaka.
Taylor also denies assaulting the child on July 2, 2015, the day before the alleged murder.
'Six blood stains'
ESR forensic scientist Gary Gillespie examined the Truman Rd property and Ihaka's clothing and bedding after his death.
He found six separate blood spatter stains on a wall beside his cot, a windowframe and curtain, as well as blood on a duvet inner, pink blanket, bare mattress and mattress protector inside the cot that matched Ihaka's DNA profile.
Blood was also detected on the child's vest, sleepsack, and baby security blanket.
A "probable bloodstain" was found on the upper front left of Taylor's T-shirt where the major contributor to DNA linked to Ihaka.
Under cross-examination by defence counsel Phil Shamy, Gillespie accepted it was possible that if Ihaka had some blood on him and then Taylor had held him, it could have been transferred.
Gillespie said he couldn't age the blood on the walls, nor could he say where any assault on Ihaka had occurred.
Under questioning by Shamy, Gillespie said many bloodstains or hair found at the scene were taken as samples but were not examined any further.
None of Stokes' clothing was taken away for examination, Gillespie confirmed.
The decision on what samples to examine further was taken in conjunction with police, Gillespie told Crown prosecutor Mark Zarifeh in re-examination.
The trial, before Justice Cameron Mander, continues.
