He found a small, painless lump in his neck and tests found he also had lymphoma across his chest.
After receiving treatment on the NHS at University College Hospital in London, including four monthly cycles of chemotherapy and heavy steroids, he is in remission having suffered severe side effects including hair loss, vomiting and fatigue.
His mother said: “We dealt with it by carrying on and trying to keep things as close to the routine that we had beforehand and also being very open.
“The day that he was told, and I think that probably us telling him is one of the toughest things that we have ever done, the following day he went to school, he stood up and he told everybody ‘I have cancer’.
“That’s the way he dealt with it but other children and other families deal with it in a different way, you have to find your way.”
Mr Clegg said: “His lymphoma was all over his chest and his neck and he gets tested every three months, I think for a couple of years, so there is always a slight spike of anxiety with us every three months but basically he is on the road to recovery.
“Interestingly the thing he was most concerned about was sort of falling behind his classmates. His anxiety was more about keeping up with his classmates, keeping up at school. So it was very impressive actually.”
He added the couple’s other children, Alberto and Miguel, had taken the news well.
“Once they heard from us that he will be OK, again they are just so, so practical – just ‘OK then’,” he said.
The couple are raising awareness of the charity Bloodwise, which will launch a report on Wednesday urging more research into less toxic treatments for children with cancer.
His wife said: “(With) chemotherapy they poison your body so that you can get cured and it’s a shock to see it happen.
“We do realise how incredibly lucky we are both with the fact that the treatment has worked and how well he seems.”
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The charity said blood cancer is the most common among children and young people, with more than 1,100 of those under 24 diagnosed in the UK every year.
Dr Alasdair Rankin, director of research at Bloodwise, said: “The reality is that one in five children diagnosed with the most common type of leukaemia still do not survive, and that those who do often experience devastating side effects both during and after treatment.
“This is simply not good enough. We need to save every child’s life, make the treatment process much kinder and give them the life they would have had without cancer.”