Ryan Ferguson, from East Kilbride, became the poster child for a national campaign for donors when he contracted Acute Myeloid Leukaemia in August 2011 aged just two years old.
Blood cancer charity Anthony Nolan searched worldwide bone marrow registers and his story sparked hundreds of people in his home country to come forward and be tested to see if they were a match.
Ryan survived thanks to a successful transplant carried out at the Royal Hospital for Sick Children at Yorkhill, Glasgow, in January 2012, but the identity of the donor remained a mystery until now.
Because of rules that allow donors to be named with their consent two years after they provide material for transplant, Ryan's parents, Paula and Stuart, have finally learned that their son owes his life to German doctor Eike Tjarks.
Dr Tjarks, a paediatrician, joined his country's bone marrow register when he was helping to recruit donors during an appeal for a young local girl who had leukaemia. He said he was delighted that Ryan was now fit and healthy.
Mr and Mrs Ferguson have been in email contact with their son's donor and are hoping to meet him in person in the near future.
Mr Ferguson said: "It was just incredible to be able to say thank you to the man who saved our son's life - what an amazing person he is. The donor has children and is a doctor, so he can appreciate what we were going through."
Dr Tjarks added: "We have thought a lot about our unknown bone marrow recipient over the last two years and always hoped that he made it and he's fine.
"We were all hugely delighted when we heard that Ryan is a healthy, happy five-year-old boy with a great family."
Ryan underwent chemotherapy while he was waiting for a donor and was left gravely ill while the campaign to find a match was being run.
During the campaign an image of Ryan featured on Anthony Nolan charity collection boxes sent to members of the public across the UK.
The appeal delivered a huge boost to the bone marrow register in the UK after his family teamed up with the charity to organise a donor drive that became the most successful ever held in Scotland.
More than 1000 people from across the country queued up to fill in a short questionnaire and provide a saliva sample as potential stem cell donors.
Strathclyde Fire and Rescue Service also joined the appeal and helped arrange a donor recruitment event at a hotel.
The Ferguson family went on to become fundraisers and have helped gather tens of thousands of pounds for Anthony Nolan to help other people in need of a bone marrow transplant, with Mr Ferguson running the London Marathon in 2012 and 2013.
The family has now emigrated to Australia, where they dreamed of living before Ryan contracted the disease.
Anthony Nolan established the world's first bone marrow register and has been helping save lives for four decades by matching people willing to donate their bone marrow to patients in desperate need of a transplant.