A man has been given a 10-year prison sentence for the assault of a five-year-old boy who suffered a traumatic brain injury, fractured ribs and skin injuries, including burns and cuts.

The 22-year-old man appeared at Parramatta District Court today via audio-visual link and was sentenced to a non-parole period of seven years and six months.

He had pleaded guilty to charges of grievous bodily harm to person with intent and assault occasioning actual bodily harm.

The man was arrested in August 28, 2020 after emergency services attended a home at Cabramatta in Sydney's south west following reports of an assault.

The court heard the man got married in June 2018 when he was 18, and that by March 2020, he was having an extramarital affair with the 31-year-old mother of the child.

While he didn't live full-time with her, the court heard that, as of March 2020, the boy referred to him by the Vietnamese word for "Dad".

Between August 22 and 28, 2020, the boyfriend assaulted the boy using his hands, arms and a broomstick with the "intent to cause grievous bodily harm", according to Judge Andrew Colefax.

The man was arrested on Friday, August 28, and the boy was taken to Liverpool Hospital and later transferred to the Children's Hospital at Westmead, where he remained for two months. 

He suffered extensive injuries including a traumatic brain injury which caused various neurological deficits.

"The police found (the mother) in the bathroom. She was holding (the boy) in her arms. He was visibly bruised and cut. His eyes were open but he appeared semi-conscious," Judge Colefax said.

The sheet on the mattress in the room was covered in splintered wood, from an item such as a wooden broomstick.

"Through your counsel's written submissions you have expressly admitted that the injuries involve the use of that broomstick.

"The police found the boy's blood on the sheet on the mattress, on the walls of the room and on the floor. A blanket with his blood was found in the laundry and a top sheet and pillowcases also with blood on them were found in the outside bin."

When handing down the sentence, Judge Colefax took the man's background into account which included being sexually abused by his aunt who was his caretaker when he came to Australia as a teenager.  

However, the court also heard the offender lacked remorse.

"I am not satisfied that you are genuinely remorseful for your offending and evaluate your prospects of rehabilitation as guarded," Judge Colefax said.

The offender will be eligible for parole on February, 27, 2028.