The grieving mom of a 17-year-old Brooklyn boy who was stabbed to death during a fight over a girl defended her son Thursday after authorities said he started the fatal confrontation.

“That’s my son!” the mother of slain teen Alex Colodner wailed outside the family’s apartment. “They can’t say anything bad about him. Nobody can! Nobody can!”

Too distraught to speak further, she was helped back to her fifth-floor apartment by another woman.

A neighbor in the building railed against cops for not charging Colodner’s accused killer, who claimed he only acted in self-defense when the larger teen attacked him.

“I don’t understand how he doesn’t get charged with anything,” the fifth-floor resident said. “A young man is dead. Somebody should be held responsible.”

Colodner was stabbed multiple times shortly before 4 p.m. Tuesday on a Bedford-Stuyvesant street after cops said he confronted the other teen for flirting with his girlfriend.

Colodner, who cops said outweighed the other teen by more than 100 pounds, then began to pound the smaller boy and was “getting the best of him,” NYPD Chief of Detectives James Essig said at a press briefing on the incident.

That’s when the other boy pulled a knife and fatally stabbed the larger teen.

Colodner was rushed to Kings County Hospital in critical condition and died the following day.

Police later declined to prosecute the other teen, ruling that he acted in self-defense.

Neighbors recalled Colodner as “a big, soft giant.”

“He was a quiet person,” one neighbor said Thursday. “I just saw him with the girl in the building. They were holding hands. That was his girlfriend. I had never seen him with a girl before.”

Said another building resident, “He was always by himself until I saw him with that girl.

“I said to myself, ‘Oh, he’s got a girl now,” he said. “But he was still a child.”