Ryan Robert Barry Hodson was charged with the manslaughter of the 21-month-old with Mason's mother Anne Maree Lee and stepfather William Andrew O'Sullivan.

Last week it was revealed he would plead guilty to a downgraded charge of child cruelty.

But he reversed his decision on Thursday once Brisbane Supreme Court Justice David Jackson questioned its legality.

Hodson walked from court a free man after crown prosecutor Vicki Loury conceded she was unable to substantiate the charge against him.

Hodson was 17 when he moved into O'Sullivan's home at Caboolture in late May 2016, a few weeks before Mason died.

Justice Jackson said the cruelty charge arose because he was left alone with the toddler on three occasions on June 10 and failed to seek medical attention.

Mason's body was handed to paramedics over the fence about 12.45am on June 11.

It is estimated he died between two to four hours earlier.

Evidence given during a committal hearing revealed he died as a result of a rupture to his small intestine, believed to have been caused by blunt force trauma up to five days earlier.

Justice Jackson said in order for Hodson to be charged and convicted of child cruelty, he had to be satisfied Mason was in his "lawful care or charge".

He said the teen was a boarder at O'Sullivan's and there was nothing to suggest he agreed to take care of the 21-month-old or did anything like change his nappy.

"It seems to me this is potentially a dangerous precedent," he said.

Justice Jackson said if Hodson was convicted it could pave the way for siblings in other households where a child is harmed to be charged with cruelty for failing to seek medical assistance.

Crown prosecutor Vicki Loury argued Mason was in Hodson's legal care because he was "an adult alone with the child".

"He was not an adult, that's critical in this," the judge fired back.

"It is time that people started talking in legal language that is accurate."

Ms Loury said Hodson was still capable of taking responsibility and had since admitted he had lawful care for Mason.

"You say because he admits it, that's good enough? It doesn't matter that in law it's wrong," Justice Jackson said.

The court heard Hodson's girlfriend claimed she observed Mason looking pale in the days before his death and urged O'Sullivan to take him to hospital.

She allegedly told Hodson she was going to ring child safety four hours before paramedics arrived, only for him to say it had "nothing to do" with them.

He claimed he saw Mason "blue on the floor" about 8.45pm on the night he died, but left the house 30 minutes later.

Justice Jackson said while Hodson could be described as uncaring and irresponsible, there was no evidence to suggest he hurt the toddler or was responsible for him.

Lee and O'Sullivan are yet to face trial for manslaughter, and Hodson remains a witness in the case against them.

Australian Associated Press