Sean and Lisa Robb said they rushed their son Harry to a minor injuries unit in Ipswich, Suffolk, after he developed severe breathing problems on June 3.
But he was sent home with penicillin after doctors wrongly diagnosed him with tonsillitis. His condition worsened during the day and his frantic parents called an ambulance
Harry was rushed to Ipswich hospital where he continued to deteriorate and two days later he was sent to the Evelina Hospital in London.
There medics told Sean and Lisa were told their son had dilated cardiomyopathy and that his organ was operating at just 17 per cent of its normal function. The condition means he desperately needs a heart transplant.
Mr Robb said: 'It is devastating. He was critically ill and they (Riverside Clinic) sent us away telling us he had tonsillitis"