Frank Rooney asked to be tested as a possible donor the day his little boy John Paul was born with an extremely rare kidney problem.
Three years later the father and son from Hamilton, South Lanarkshire, had the transplant surgery and, just two weeks on, they are recovering well.
Mr Rooney, 30, who owns his own business, said: "People always say you would do anything for your kids and it's true.
"When I think of the amount of pain he has gone through, two weeks of pain for me is nothing."
John Paul was diagnosed with kidney problems in the womb. John Paul, Mr Rooney and his partner Maryanne's first child, had to be delivered a month early so he could receive emergency medical treatment for his prune belly syndrome - a rare genetic disorder that affects the kidneys and bladder.
Babies with the illness, have no abdominal muscles or very weak abdominal muscles and can be born with extra wrinkled skin on their bellies.
Since being allowed home at three weeks old, John Paul has needed round-the-clock care, including dialysis at home every night and weekly visits to the Royal Hospital for Sick Children at Yorkhill, Glasgow.
It had been hoped that his condition, which affects just one in 40,000 births, would be managed by drugs but doctors realised a transplant would be necessary.
His mother and father were tested for compatibility and Mr Rooney was found to be a 99% match.
Mr Rooney said: "I thought I would be able to do it immediately but they said we would have to wait until he was bigger. The process was a lot simpler than I thought. The tests are meant to take around nine months but I took lots of cancelled appointments so I had all the tests in seven weeks."
The couple have raised almost £5000 for Yorkhill to say thank you for the care their son has received so far.