Little Kayden Walker suffered "catastrophic" brain injuries after being shaken by Ricky Walker, 27, at home in Bilston, West Midlands.
The tot's mum Laura Davies has now been jailed for 18 months after she was found guilty of allowing her son's death.
Birmingham Crown Court heard the 25-year-old had “failed to heed warning signals” posed to six-month-old Kayden.
Judge Mr Justice Morris said she should have raised the alarm when Walker previously spoke about suffocating their son.
Walker was last month convicted of manslaughter and jailed for six years following a joint trial with Davies.
The court heard how Kayden was subjected to cruelty which led to an earlier head injury, several weeks before his death on June 12, 2016.
Kayden went into cardiac arrest at his home and was pronounced dead in hospital about an hour after Walker dialled 999.
Opening the Crown's case at the start of the trial, prosecutor David Mason QC, alleged that Walker had previously joked about how long it would take his son to suffocate when a hoodie covered his face.
David Mason QC, prosecuting, said Walker had "vigorously" shaken Kayden which caused a severe "bang" to the head on June 12, 2016.
He told the court: "The evidence in this case will show that, just before Kayden's collapse and that call to the emergency services, he was almost certainly the victim of an episode of forceful shaking, involving an impact to the head."
Walker, who had a small level of cannabis in his blood at the time of the death, told his trial he could be "heavy-handed" but denied fatally shaking the child.
Under cross examination during his trial, Walker told jurors Kayden's brain injuries "were probably through me panicking and that" but said he could not explain why he became unresponsive.
He also told jurors he believed a folded-up hoodie he used to "prop-feed" Kayden may have suffocated him.
Sentencing Davies today, Mr Justice Morris said: "You understood the significance and severity of the suffocation comment.
“You failed to take such steps as could have reasonably been expected to protect Kayden from that risk.
“You failed to raise many of these matters before June 12, 2016, and you failed to ensure that Ricky Walker was not left alone in the flat with Kayden as he was on the morning of June 12, 2016.”
An NSPCC spokesman said: “Davies would have been aware that there was a significant risk to Kayden but despite this, she didn’t take sufficient steps to protect her son – which resulted in tragic consequences.
“This case is a poignant reminder of how important it is for anyone to come forward if they are worried about the wellbeing of any child.
"Anyone who has any concerns about a child can call the NSPCC Helpline for free and confidential advice on 0808 800 5000.”
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