But the boy's mother disputes the story and says police have it all wrong.
Police admit it's a challenging case because there are no outside witnesses, but one thing investigators are sure of: The child had four bleeding puncture wounds in his thigh that looked like they came from the prongs of a fork.
A neighbor called 911 after he heard commotion in the apartment where the boy and his step-dad were playing. The neighbor sensed the boy was in trouble and called police after the boy left.
"(Officers) are told by the juvenile that his stepfather is acting drunk and acting crazy, and that he had duct-taped him, and that indeed he had stabbed him," said Aaron Snell with Everett Police.
Officers arrested Steven Williams for investigation of assaulting a child and unlawful imprisonment. The boy told them Williams stabbed him in the thigh with a fork. But the child's mom says it's a misunderstanding.
"They were rough-housing so my son went to go kick him and he had a fork in his hand and it got his leg," said the mother, who wants to remain anonymous. "And that's all that happened."
Police saw puncture wounds that were still bleeding above the child's knee. The boy also told officers that after he was stabbed, his step-dad bound him in duct tape and left him tied up for more than an hour.
The boy's mother says it didn't happen at all.
"My son and I were playing around a week or two ago, and I was playing around with the duct tape," the mother said. "I told him we were going to do that, joking around."
Williams has an extensive criminal record with past convictions for theft, drugs, and drunk driving. He also had an outstanding warrant for domestic violence assault.
The 9-year-old remains with his mom at their Everett apartment. She says the boy blames himself for his step-dad's arrest.
"He's fine physically," the mother said. "He feels horrible about the entire situation. Of course he feels like its his fault now, and I've told him it's not. He's trying to deal with it just like we all are."
Williams is being held in the Snohomish County Jail on $75,000 bail.