The child remains in a critical condition in hospital after the shooting at a home on Fairway Drive in Norwest about 5pm on Monday.
Detectives from The Hills Police Area Command have charged an 18-year-old man with concealing a serious indictable offence. Two other men, also aged 18, have been released after being questioned. A 43-year-old woman was also present, and police are still searching for another person who was at the scene.
Police initially said there was no evidence the shooting was related to organised crime, but the ABC understands investigators are now exploring whether men inside the unit had connections to criminal networks.
"We have nothing to suggest that it is organised crime-related, but we will conduct a very thorough investigation into this," NSW Police Superintendent Naomi Moore said on Monday night, before the latest developments. She confirmed the firearm had not been recovered.
Five other people in the apartment at the time were uninjured. Paramedics treated the boy at the scene with two specialist medical teams before he was taken to Westmead Children's Hospital.
Local residents described the scene as "frightening" and "something out of the movies", with police cordoning off the area and searching a nearby park and creek late into the night.
Police are urging anyone with information to contact Crime Stoppers.