A 15-year-old boy who was shot in the head "execution-style" as he lay in bed on Friday morning has died in hospital.

Brayden Dillon, 15, was shot at point-blank range after a masked gunman kicked down the door to his family's home in Glenfield, in south-west Sydney, just after 6am.

Investigators believe the shooting may be a revenge attack linked to a previous homicide.

Fairfax Media understands authorities had been called to the family's rented home in Moresby Avenue at least three times in the past two years for incidents involving Brayden.

​In one incident, he allegedly tried to attack his mother with a knife.

In another, he was allegedly smashing windows and property in the home, forcing his pregnant mother to call paramedics out of fear.

Brayden, his mother, his step-father and his step-siblings, aged one and seven, were all asleep in the Glenfield home on Friday morning when the gunman burst in.

It's believed he threatened the mother and pulled out a pistol, forcing her to flee to a bedroom to hide.

He then walked to Brayden's upstairs bedroom and shot him in the head at point-blank range as the teenager lay in bed, Detective Chief Inspector Mark Brett said.

Mr Brett confirmed on Friday morning that the 15-year-old was known to police but would not elaborate on any possible revenge motive.

He said the family were not able to identify the gunman or see his face properly.

Police are hunting for a man described as being aged between 18 and 20, of thin build, and wearing grey tracksuit paints, a navy hooded jacket and white runners.

Police urge anyone who sees this man not to approach him but to call triple zero immediately.

The 15-year-old boy was taken to Westmead Hospital in an "extremely critical" condition, but he died several hours later. 

Friends paid tribute to the teenager as news spread on Friday afternoon that he had passed away.

"I cant live without you your smile your eyes your everything about you was just absolutely perfect," one friend posted.

"I love you brayden u will forever be my one true love i cant believe this happened to you and its killing me."

Another posted: "My love goes out to you and your family you didn't deserve this shit."

Brayden's mother had been by her son's bedside and was "extremely distressed," Mr Brett said.

The step-father has taken the two younger children to another location, he said. 

A neighbour said she woke about 6am to see paramedics desperately working on the boy while loading him into an ambulance. 

"I've never seen anything like it in this area," she said. 

A man who lives two doors down said he woke to hear two gunshots.  

"I thought it was someone loading a truck, you know, Good Friday or something," he said. "When I came outside I saw police and ambulance everywhere."

He said the family moved in to the rented property about two years ago.

- with James Lemon