A six-week-old baby was "grey" and at the point of death when he was first seen by his GP, a court heard.
Jody Simpson, 24, and Tony Smith, 46, deny assaulting, ill treating or neglecting their child to cause unnecessary suffering or injury.
Jurors at Maidstone Crown Court heard they delayed taking their son, also called Tony Smith, to a GP because they were waiting to get their boiler fixed.
Expert paediatrician, Dr Kathryn Ward, told the court he was "moribund".
Ms Simpson and Mr Smith, of Sydney Road, Whitstable, called the GP on 14 November 2014, saying their son had cold symptoms.
Tony was seen by a doctor four days later at which point he was grey, had froth at the side of his mouth and his legs were swollen and hard.
He was immediately transferred to hospital, and Dr Ward told the court: "Without very aggressive medical intervention the results would have had a very different end.
"This includes the GP immediately giving him antibiotics, calling an ambulance and blue lighting him to hospital.
"It's a credit to the doctors that he didn't die. He had multiple system failure. Every system in his body was affected by toxins."
The jury previously heard his growth was normal, but X-rays at the hospital revealed the baby had eight fractures, caused by "twisting or pulling" and using "excessive force".
When asked about the pain he would have been in from the injuries, Dr Ward said: "I would suggest that a child who was in pain would not necessarily stop crying.
"You would notice that putting a leg in a Babygro was painful."
The court heard Tony also had multiple infections and septicaemia.
Three further fractures, found in a second X-ray two weeks later, are believed to have been caused by the bone infections, the jury were told.
The trial continues.