A $1 million reward is being offered for information about missing four-year-old Gus Lamont, as his parents reveal their ongoing heartache over the unsolved disappearance of their “sweet and unique little boy”.

Gus was last seen playing near his family’s home at remote Oak Park Station in South Australia on September 27 last year.

SA Police, who have since declared his disappearance a major crime, announced on Wednesday that the reward offered by the state government would be paid for information leading to the conviction of the person or people responsible for the little boy’s disappearance and/or the recovery of his remains.

“Somebody knows something,” said Detective Superintendent Darren Fielke.

“We urge… anyone who has any information that can assist this investigation to come forward.

“A million dollars is a life-changing amount of money for someone. Now is the time to come forward and take advantage of that.”

Numerous ground searches and aerial searches have been carried out at Oak Park Station, which is about 40 kilometres south of the South Australian town of Yunta. However, they have failed to find any trace of Gus.

In a video message shared on Wednesday, his parents Jess and Josh Lamont urged anyone with information to cooperate with police, saying “even the smallest detail” could be helpful.

Jess Lamont described Gus’s disappearance as “the most devastating and inexplicable event” they had ever experienced.

“We love him, and every single day without him has been incredibly difficult. Our heartache only grows as time passes and an entire year without any answers is upon us,” she said.

“Our memories of Gus are of a beautiful, sweet and unique little boy who loved music, listening to stories and playing in the natural world.”

A team of 17 officers from Taskforce Horizon and STAR Group searched the area in May. Photo: SA Police

During the past year, the family should have celebrated Gus’s fifth birthday, with his little brother also turning one.

“One of the most heartbreaking things is that Gus and his little brother will not grow up and share their lives together,” his mother said.

A visibly upset Josh Lamont thanked all those who had helped search for his son.

“We miss Gus more than we can describe,” he said.

“We are desperate to have him return home to us and we are desperate for answers. We believe that someone knows something; someone knows what happened to Gus and may even know where he is.”

Detectives believe it is unlikely that Gus wandered off from the family homestead or was abducted by an unknown person, saying a focus of investigations remains on “a person known to Gus being involved in his disappearance and suspected death”.

Fielke reiterated that Gus’ parents were not suspects.

SA Premier Peter Malinauskas said the four-year-old’s disappearance had shocked and saddened South Australians.

“I cannot imagine the anguish that Gus’s parents Jess and Josh are going through, and my thoughts remain with them,’’ he said.

“The State Government is doing everything it can to support the police investigation. That’s why we are offering a $1 million reward to help solve this case.”

In an interview this week, Gus’s grandmother Josie Murray said it ”brings me to tears that he’s not here”.

Murray said she still wanted to know what happened to her grandson.

Police had told her she was the main suspect but she believed Gus was taken by a stranger.

”I think he has to have been taken,” she said. ”Nothing else makes sense.”

After the major crime declaration, grandparents Josie and Shannon Murray released a brief statement via their lawyers saying they were devastated by the police assertions.

“The family has co-operated fully with the investigation and want nothing more than to find Gus and reunite him with his mum and dad,” they said.

–with InDaily and AAP