Ryan Robert Barry Hodson was expected to plead guilty to a single charge of child cruelty in the Brisbane Supreme Court on Thursday after it was downgraded from manslaughter.

Instead he was released from custody after crown prosecutor Vicki Loury conceded she was unable to substantiate the allegation against Hodson, who was 17 years old at the time.

It followed Justice David Jackson's questioning of the legality of the charge.

He told the court the cruelty offence arose because Hodson 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 he was not convinced Mason was in Hodson's "lawful care or charge" and as such any conviction could set a dangerous precedent.

"He was not an adult, that's critical in this," 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.

He 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.

Mason's mother Anne Maree Lee and stepfather William Andrew O'Sullivan are yet to face trial for manslaughter, while Hodson remains a witness in the case against them.

Australian Associated Press