Matthew Pietrzyk, who had neither of his kidneys, was constantly on dialysis and took 18 tablets a day just to stay alive.
The youngster, from Glenfield in Leicestershire, was on the UK transplant waiting list for eight years before he received his “gift of life”, his family said.
Leicestershire man, Edward Batch, 39, decided to support the family in a running event by raising awareness for organ donation through wearing a special T-shirt. But when he heard more about Matthew’s plight, he decided to sign up to become an altruistic organ donor.
He posted online: “I don’t know if it is as easy as this so excuse my ignorance but I have a spare kidney if it helps; I’m more than willing to donate to a good cause.”
The operation took place around two weeks ago at Birmingham Children’s Hospital.
Matthew’s mother Nicola Pietrzyk said: “Matthew has finally found his hero! It is so wonderful to be able to say that his day finally came after such a long wait.
“Matthew was antibody sensitive to 99% of the population — but we never gave up hope. It’s still very early days after the operation, but Matthew is doing really well so far.
“We cannot thank his donor and family enough for this incredible gift. This is the transplant we have been waiting so long for. It will change Matthew’s life and life for our whole family.”
Batch, who is married with three children, said it was an “honour” to help the family.
“I just felt I wanted to offer to try and help. I contacted the transplant team and said I wanted to volunteer as a living donor — I’d been prompted by Matthew’s appeal but if it didn’t turn out to be a suitable match I would have been a donor to another patient in need if I was suitable.
“Throughout the testing process I only had a little contact with Matthew’s dad, just to keep him updated. I was very aware it was long odds that I would be a suitable match.
“But once we were told it was 100% certain the donation and transplant could go ahead, we did all meet up shortly before the operation.
“I had always believed in organ donation, but hadn’t been aware of living donation.
“I’m a dad and I’d like to think if any of my children were in need someone would step forward to help and it was an honour to be able to help another family in this way.”
Pietrzyk said: “While we look forward to celebrating Matthew’s transplant, we hope that hearing of his good news will prompt more people to think of the gift they could give others” by signing up to be an organ donor.
Matthew received a kidney from his mother Nicola when he was two but it was removed after just two days following complications.
When he was a baby he was diagnosed with a rare kidney condition called congenital nephrotic syndrome and doctors said Matthew would not survive childhood without a transplant.