Jason Takor had his right testicle removed and underwent rounds of chemotherapy and radiotherapy that caused his hair to fall out.
The highly unusual diagnosis came after his mother, Zuzana, discovered a lump the size of a tangerine. Jason endured gruelling treatment and went into remission in October last year, when he was 4 years old. He now has check-ups every three months.
Takor, 34, of Bristol, said: “When I first found out, it felt like a massive black hole opened up.
“I tried not to think about what might happen.
“I never cried in front of Jason. He was remarkably positive.”
Jason said: “The medicine made my hair fall out, but now I’m just looking forward to getting dinosaurs and Lego for Christmas,” she said.
Professor Thomas Powles, a testicular cancer expert for 20 years, described Jason’s diagnosis as a “one in a billion or more” case.
“Genetic factors could contribute to a boy developing testicular cancer at this age (but) it is not something parents of pre-school age children need to worry about,” he said.
Cancer of the testicles most commonly affects men aged between 15 and 45, with around 2 200 cases diagnosed each year.
Almost half of men diagnosed are under 35.
Despite it being a relatively rare form of cancer, it is one of the most treatable. About 98% of sufferers who spot the disease early are disease-free after treatment. If caught at any stage, 96% of men will still be alive 10 years after their treatment.
Unlike many cancers, there are few known risk factors for testicular cancer.
Men born with an undescended testicle, are at a slightly greater risk and around 10% of sufferers will have a history of this condition. Having a brother or father affected by testicular cancer can also increase a man’s risk.
Some research has suggested that the disease is slightly more common in men with fertility problems, and men with HIV are also more likely to develop it.
In 2015, 9-year-old Jack Bristow was diagnosed with the disease. At the time, doctors thought he might have been the youngest in the world. He discovered the condition by chance, when he was taken to the doctors after knocking his groin playing football at school.
There were 2 418 new cases of testicular cancer in the UK in 2014, and 60 deaths as a result of the condition.