Harry Connolly's parents begged for their son to be kept in hospital after he was admitted suffering severe diarrhoea and dehydration, an inquest heard.
But doctors there failed to carry out vital tests on the toddler, instead dismissing his illness and advising his parents, Lucy and Raymond, to take their son home.
A fourth doctor, from a GP out-of-hours service, also failed to spot that Harry was seriously ill or refer him to hospital for treatment despite the fact the little boy appeared 'lifeless'.
Instead Harry died at home in his sleep, four days after he was first taken to hospital. A post-mortem examination revealed he died of dehydration and acute kidney failure after suffering from an inflammation of the colon, which had not been spotted.
Coroner Anne Pember criticised doctors at Northampton General Hospital and a GP's out-of-hours service.
She said he could have survived had he been re-admitted to hospital and given proper treatment.
Last night Harry's parents, who have two older sons, said he had died needlessly because of "sub-standard" care.
Lucy, 29, told reporters: "I kept telling the medical staff that Harry was extremely sick but nobody would listen. Three opportunities to save my son were wasted."
Harry's parents are suing Northampton General Hospital and have lodged complaints to the General Medical Council about two of the doctors involved. The hospital has apologised and said changes had been made.