James Morley, 13, suffered from breathlessness and lethargy and made repeated visits to doctors but despite carrying out tests, they could find nothing wrong.
It was only when James complained that he was seeing everything with a pink tint that his mum, Pamela, 47, booked an appointment with an optician in their home town of Sutton-in-Ashfield, Nottinghamshire.
There, optometrist Sunny Boyal found that James had a large haemorrhage and oedema (a build-up of fluid) on his left eye.
Pamela said: "As soon as I told the optician what James' symptoms were they said I must bring him down straight away."
James was immediately referred to King's Mill Hospital, in Mansfield, where the consultant confirmed the bleed and sent him to a paediatric ward.
There he was told he was suffering from a form of leukaemia and was admitted to the Queen's Medical Centre in Nottingham.
Pamela said: "As soon as that doctor said something was not right I was singing hallelujah - I knew all along that something wasn't right and it had been so stressful to be told over and over that he was fine.
"As a mother you instinctively know when your child has something seriously wrong with them.
"All his life James had been so hyperactive; he was bouncing off the walls most of the time so we were worried about him when he suddenly changed and was out of breath, tired and getting temperature spikes.
"I wasn't prepared to just accept the doctors' word that James was fine. And it's lucky that I did keep pushing for an answer - because the earlier you find the cancer the better the chance of survival."
James was eventually diagnosed with Philadelphia positive acute lymphoblastic leukaemia - a rare illness in children.
Pamela said: "When James got his diagnosis it was a huge shock for the family, but thanks to Sunny we were able to catch the leukaemia in the early stages so were stayed positive."
As part of James' two-year treatment, which included normal chemotherapy, he became the first child in the UK to trial the drug Dasatinib - which only became available for trial on children immediately after diagnosis.
Emma Astwood is the paediatric haematology consultant at Nottingham Children's Hospital.
She said: "Acute lymphoblastic leukaemia is the most common type of leukaemia, but Philadelphia positive only accounts for three to five per cent of child cases.
"It means the leukaemia is more difficult to treat than normal."
But after successful treatment James' mother, his father Martyn and younger sister Adele, seven, can look positively to the future.
Pamela said: "Things are going well and he is classed as in remission now. He could relapse but we are carrying on with his treatment. It will be five years before he can be classed as in full remission.
"Our experience really has opened my eyes to how important it is to make sure your children see an optician regularly - I didn't realise the kinds of conditions that they can pick up.
"It's not an exaggeration to say that Sunny saved my sons life. He's a proper hero."
James and his family have been supported by the charity when You Wish Upon a Star, which aims to grant the wishes and special VIP treats to children suffering from life threatening illnesses.