Archie Eldridge had been given a temporary cast after an X-ray revealed he had a hairline fracture following an accident in gymnastics.
Ten days later, mum Natasha, 33, took Archie to a specialist who inspected the X-rays and then removed the cast.
"The doctor informed us there was no fracture to his leg and asked Archie to walk, but he struggled to put any weight on his right leg," Natasha, from Dartford, Kent, told the Sun.
"The doctor said this was psychological and he would be walking within the next few days."
But two weeks after being discharged from Darent Valley Hospital, Archie was still in pain and struggling to walk so Natasha took him to another hospital for a second opinion.
Staff at Queen Mary's Hospital in Sidcup referred the youngster to his GP, who then sent him back to the SAME fracture clinic that had made the mistake.
After more X-rays, the consultant admitted he had misdiagnosed Archie two weeks earlier.
On returning to the consultant at Darent Valley we were told he had a definite break just below the right knee," said Natasha.
We were told Archie was very lucky there was no long-term damage.
"He admitted the initial fracture diagnosis was a mistake and we had every right to be angry but we should concentrate on getting Archie back to full health."
A Darent Valley Hospital spokesman said: "We take complaints very seriously and we will be investigating.
"Once we have concluded our investigations, we will be writing to Mr and Mrs Eldridge with the results of our findings."
Poor little boy!