A 22-year-old survivor of a Queens house fire that killed a woman and her newborn baby was fighting for his life Wednesday, with severe burns throughout his body after going back into the inferno to rescue his mother, who had already escaped, his brother said.

Doctors said Thet Aung Oo suffered burns on 99% of his body, and relatives said his lungs were filled with smoke amid worries that his heart and kidneys might fail.

“These next four to five days are very critical for him,” said the victim’s brother, Phyo Ko.

He said doctors told him that his brother may undergo another surgery, and that he is going to need ventilation for a long period of time.

Oo was critically injured Monday night when a fast-moving fire swept through a subdivided Elmhurst home, killing Miguelian Alcantara, 34, and her newborn baby, Emma.

The intense fire at the home on Dongan Ave. near Broadway spread to a neighboring building and destroyed it.

Ten survivors needed medical treatment, including Oo and three firefighters, who suffered minor injuries, officials said.

“We don’t know the cause, but we believe it started from the basement,” Ko said of the blaze. “That building is very old, and there have been fire issues, like, three or four times already. We informed the landlord, but they haven’t taken much action. Four months back, there was a small fire incident in the kitchen on our floor, and that was taken care of, no injuries. Nothing to this degree.”

He said his brother and their parents lived on the first floor.

“My parents started smelling the smoke, so my mother went out to find out what happened,” Ko said. “Once she opened the main entrance door, a lot of smoke came into the room. My mother managed to get out of the building, but my brother didn’t know whether she was in the building or not. Confusion happened, and he went back in.

“We saw him outside,” Ko said. “He was naked, and his body was completely on fire. He started rolling in the snow, ice and then the firefighters carried him to the ambulance.”

Ko said his father suffered a minor injury to his back, and his mother burned her hand when she tried to help her burning son.

Ko said there were three or four bedrooms in the basement housing people from different families, including the mother and daughter who died.

“We had casual conversations, but I didn’t know her well,” he said of Alcantara.

Ko said his family doesn’t know what to do.

“My parents are devastated, but I have to stay strong,” he said. “I’m just holding on, staying hopeful. We haven’t let my mother see him because I know she’s going to have a breakdown seeing him in this condition.”

A GoFundMe page by the fiancée of Oo’s boss noted his selfless act, and called him courageous for putting his mother first. Ko said his brother worked in a small jewelry company in Manhattan’s Diamond District.

“They are like family, and they are helping him,” Ko said.

He said Oo and his family were planning to move to Florida to jumpstart a new jewelry store with his boss.

“They were looking for an apartment there, and then this happened,” he said.