The body of Julian King was lying in the back seat of the white car they had been looking for since the shooting of the actress's mother and brother in Chicago on Friday. The child had vanished from the house in the wake of the double murder, prompting a city-wide hunt for the boy.
Ms Hudson identified the boy's body at the Cook County Medical Examiner's office Monday afternoon.
The original alert for Julian, the son of Julia Hudson, the actress's sister, said that he may have been snatched from the house by someone driving a 1994 white Chevrolet Suburban.
On Sunday, Ms Hudson, who soared to fame with her role in the 2007 musical film Dreamgirls, issued an appeal to the public in her MySpace blog for help in returning Julian. She also offered a $100,000 (£63,000) reward for his return. Julia Hudson, who was at work as a school bus driver at the time of the murders, had made a similar plea to friends and neighbours on Saturday. Julian had been the subject of a frantic search since the killings, which Chicago police have characterised as "domestic related".
Authorities have been questioning Julia Hudson's estranged husband, William Balfour, 27, who is held in state custody on a parole violation relating a previous conviction, when he served seven years in prison for attempted murder. Balfour is the stepfather of Julian. No one has yet been charged for the killings and the motive remains unknown. However, officials said that Mr Balfour remained a "person of interest" in the investigation.
The bodies of the first two victims, Jennifer Hudson's mother Darnell Donerson, 57, and her brother, Jason Hudson, 29, were found by a family member later on Friday in the house that belonged to Ms Donerson and was home to several members of her extended family. By then, Julian was already missing.
The mother of Mr Balfour has publicly insisted that her son was no longer involved with Julia and had nothing to do with the murders of the brother and mother.
However, police sources told The Chicago Sun-Times that he remained their "primary suspect", saying they had tracked his whereabouts on Friday from mobile telephone records. Mr Balfour's girlfriend, who lives at the house where he was arrested, is believed to have contradicted his alibi.
A neighbour of the girlfriend told the paper that he recalled seeing a man matching Mr Balfour's description drive up in a white SUV and park on the block late on Friday morning – at least two hours after the slaying. The man got out of the car carrying what appeared to be a bottle of spirits and walked into the home where Mr Balfour was arrested later in the day.
The tragedy is an extraordinary reversal of fortune for Ms Hudson, who first found fame as an also-ran on the reality talent show, American Idol. She went on to win the Oscar for supporting actress in Dreamgirls. She had been preparing to promote a new album this week and her new movie The Secret Life of Bees has been well received and reviewed.