Ramel Echols, 28, of Milwaukee, US, is in custody after Ky’mir Williams suffered massive brain injuries.
Police said the baby had haemorrhaging similar to ones suffered in high speed road collisions.
He also suffered terrible bruising in his abdomen, doctors said.
Family members told Fox6 News on Monday: “Their heart will forever be broken.
“He was a bundle of joy to our family.
“I hope that whoever was involved gets in trouble.
“He didn’t deserve that… he didn’t deserve that.”
According to a criminal complaint, Echols noticed his son was “throwing up blood and milk” and called the baby’s mother.
When she told him to call 911, police say he waited, eventually taking his son to the hospital.
The boy’s injuries were so serious, doctors say they were “consistent with an injury seen in a high-speed motor vehicle accident”.
He was also said to have “bleeding on the brain” and “bruising to the abdomen.”
Investigators said Echols offered no explanation for the injuries.
The baby was placed on life-support and taken off of it last Friday.
Echols has been charged with child abuse and neglect, with homicide charges expected.
It comes after footage of a woman pushing a panicked toddler in a swimming pool and watching as she cries was uploaded on TikTok, with the comments disabled.
Shannon, who calls herself a “paediatric drowning prevention specialist”, has previously received messages from “trolls” telling her she is terrifying kids.
But the mum says she is teaching tots to float on their backs so they do not drown if they fall in a pool.
In the clip, which she uploaded on her @whynot0818 TikTok channel on October 14, Shannon pushes a baby wearing a pink coat into the water.
The frightened-looking youngster immediately begins to flail her arms and ends up on her side, with her face half-submerged in the water.