The mum and dad, from King's Lynn, Norfolk, told The Sun that their boy's weight gain was genetic and they have tried to keep his size under control.
His 43-year-old mother said: "The idea that they could take away my son scares me so much. I'm nothing without my kids. I am chubby and the whole of my husband's side of the family is big. It's genetics. You can be genetically fat."
The working dad, 49, and his wife said they encourage their son to play sport by giving him toy guns to fire in the garden.
They insist he is on a strict diet - and that he keeps active with regular sessions on computer games including his Nintendo Wii.
The pair signed up to a voluntary plan to tackle his weight when they were threatened with legal action earlier this year, but it brought no improvement.
The dad said: "He's always been big. He was born with shovels for hands and spades for feet. Everyone on my side of the family is big. There's nothing we can do about it. I only eat occasional kebabs. Our son's favourite snack is steamed broccoli - and he's still big."
The boy's ballooning weight was spotted by teachers at the specialist school he attends for help with autism. The parents agreed to see a social worker.
But doctors at the local Queen Elizabeth Hospital called police when he was brought in for treatment twice during March.As he was already known to social services, specialist police officers visited the family's home and asked to see the boy.
The mum and dad were asked to voluntarily attend a police station. The dad visited that day and was kept in a cell for 50 minutes before being interviewed under caution on suspicion of neglect and child cruelty.
His wife was interviewed under caution on suspicion of the same offences the next day. Both parents were freed on police bail.
And at a meeting with social workers, the couple agreed to a letter of intent over his care.
If they fail to stick to the terms, legal proceedings will be started to take the child into care.
A Norfolk County Council spokesman said the authority could not comment.