The incident happened at the boy's home on the 200 block of South Carondelet Street around 9 a.m.
Police say the boy's 26-year-old stepfather used a kitchen knife to stab Lewis several times. The boy was in cardiac arrest when he was transported to Children's Hospital Los Angeles. He succumbed to his injuries shortly after.
Neighbors say they called authorities when they heard Lewis' mother pleading for her son's life.
The unidentified man was in a romantic relationship with the boy's mother and reportedly lived with them. He was stabbed in the shoulder by the boy's mother as she tried to protect her son.
He remains under guard at County USC Medical Center and will be booked on charges in connection with the boy's death upon his release.
No word on the motive has been released. The investigation is ongoing.