A Kings Park man who was convicted of killing his infant son is asking for the verdict to be thrown out after a woman has offered evidence suggesting the baby’s mother killed the boy.

Christopher Foster, 32, was sentenced last year to 20 years and 42 days in prison for first-degree manslaughter in the death of 42-day-old Jonathan Hertzler.

At his trial, Foster’s defense suggested that the boy’s grandmother, Barbara Hertzler, a schizophrenic who suffered from hallucinations, killed the boy at their Ronkonkoma home on Oct. 11, 2011.

But now Carol Stalhut, a friend of Hertzler, has offered a different version of events. In a statement submitted with the defense motion to set aside the verdict, Stalhut of Ronkonkoma said Hertzler tearfully “asked me to keep her secret.” That secret was that she saw her daughter Clarissa, Jonathan’s mother, suffocate the baby while she was high on drugs.

Clarissa Hertzler, 31, testified at Foster’s trial that she initially believed her boyfriend was innocent but after an autopsy showed skull fractures and other broken bones, she came to believe that he did it. She also acknowledged a persistent addiction to opiates.

After the baby’s death, she was charged with drug possession and driving while influenced by drugs or alcohol. She pleaded guilty to the drugged driving charge. She could not be reached for comment Thursday, and her attorney did not respond to a request for comment.

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The Suffolk district attorney’s office said there’s no reason to throw out the conviction.

“The defendant’s application is based upon inherently unreliable, untrustworthy and inadmissible statements from a witness with a long psychiatric history, including hallucinations,” spokesman Robert Clifford said in a statement, referring to Barbara Hertzler.

“Moreover, the statement [from Stalhut] conflicts with the trial evidence, including the medical examiner’s testimony that . . . Jonathan Hertzler died from blunt force trauma including skull fractures — not suffocation,” Clifford said.

Defense attorney David Besso of Bay Shore said Barbara Hertzler’s psychiatric issues don’t necessarily negate her value as a witness.

“You don’t have to be crazy to see if your daughter’s killing the baby,” Besso said.

He also said the baby’s injuries, which included skull fractures on both sides of his head, were consistent with his head being squeezed during suffocation.

“I think the conviction has to be vacated,” Besso said.

The defense motion asks the trial judge, state Supreme Court Justice William Condon, to set aside the verdict. If that happens, there would be a new trial.