The mom of the Brooklyn 14-year-old fatally shot by his 12-year-old cousin wept Tuesday as she told The Post about her “beautiful’’ boy — and how his clean clothes are still folded on his bed, waiting for him to put them away.

“My baby was everything,” said Tiffany Grant, 35, at a growing memorial outside her mother’s Dean Street home in Crown Heights, where her son Jasai Guy spent part of his youth.

“No one is perfect, but he was very close to it — maybe that 1% of it was when he didn’t want to do the dishes,” she chuckled through tears.

“His books are at home in his room on a bookshelf,’’ she said sadly. “His clothes that I washed are on his bed waiting for him to put away.”

The mom’s heartbreak was revealed as authorities released grisly new details about the shooting — including how Jasai was shot through the heart by an illegal, sawed-off shotgun.

Jasai and his younger cousin were playing at their paternal grandfather’s home, where the victim’s firefighter dad also lives, on Osborn Street in Brownsville around 10:30 a.m. Sunday when a shotgun believed to belong to the father went off, police and law-enforcement sources have said.

Jasai was hit in the chest — and the younger boy later told cops in a heartbreaking admission, “I shot my cousin by accident, I don’t want him to die,’’ according to the sources.

The 12-year-old told authorities “that he was trying to take the gun away from his cousin and it went off,” NYPD Chief of Detectives Joseph Kenny told reporters Tuesday.

As for the scene, “there’s blood in the apartment hallway. There is a gun, a shotgun on the bed [inside],” Kenny said. “There is blood on that bedroom wall and shotgun pellets embedded in the wall. The shotgun itself is altered.

“Both the barrel and stock were cut off to make it smaller. It’s a Mossberg model 518 12-gauge shotgun,” the top cop said. “We have one spot spent shell casing at the scene and five live rounds inside the shotgun.

He added that doctors described Jasai’s injures as “a gaping chest wound and a hole in the victim’s heart.”

Both the 12-year-old and Jasai’s father have gotten lawyers and are not speaking with authorities further, Kenny said.

Grant said she lost an adult brother five years ago and was supposed to visit his grave on the anniversary Tuesday — when she is now preparing to bury her son.

“My boy was so beautiful,’’ she said.

The shattered mom said her son was extremely bright and creative, a kid who loved to draw comics but also play basketball.

“He was the ‘Cool guy Sai,’ that’s what they called him,’’ Grant said.

“Saturday, he just got his contacts,’’ the mom said. “He was excited. The pictures you see where he didn’t have his glasses, he couldn’t see.

“He played basketball, and they kept breaking every week. He got tired of us yelling at him about breaking his glasses. So he eventually was like, ‘OK, Mom, I’m ready to try contacts,’ ” she said.  

Among the messages written around pictures of Jasai at the sidewalk memorial were some from Grant.

“Mommy loves you with my all and everything. I couldn’t hold you in my arms forever. But now I can hold you in my heart forever. I love you my first born, my king, my forever love,’’ the mom wrote.

Grant said Jasai’s little sister, Kennedy, 7, “has her moments” grappling with her big brother’s death.

“Her main thing is she calls herself ‘the comfort helper.’ So I asked her what does ‘the comfort helper’ do, and she says they look around for people that’s crying and give them hugs,” the mom said.

Jasai’s cousin, who has not been publicly identified because he is a minor, was charged with criminally negligent homicide and criminal possession of a weapon in the death.

He appeared in Brooklyn Family Court on Monday and was freed to his mother, pending his next hearing Friday.

Jasai’s father was in Costa Rica at the time of the shooting, but his grandfather was at home sleeping, authorities said.

“I don’t believe the grandfather was aware that a gun was in the house,” said NYPD Deputy Commissioner of Public Information Tarik Sheppard during an interview with WPIX-TV on Tuesday.

It is unclear whether an adult will be charged over the gun, which appeared to be in the father’s bedroom, law-enforcement sources have said.

“This is just the case where they’re messing around with the gun inside of the apartment. It goes off, it costs the loved one their life. … We are extremely empathetic to that family,” said NYPD Deputy Commissioner of Operations Kaz Daughtry in the same interview.

Such cases are “one of my biggest nightmares,” Daughtry said.

“That’s why I’ve been doing this since, for almost 10 years now, when my first kid was born. As soon as I get home, I put my gun right in the safe,” the police official said. “I don’t put it on the table. Muscle memory, right to the safe, because God forbid I wake up and leave my weapon on the table and something like that happens.”

Grant told The Post, “If the world needs to know anything about my son, I don’t want the last thing to be that he was shot dead.

“It’s beside that, it’s who he was. And I’m going to tell that story.” 

Additional reporting by Amanda Woods