His condition got worse until his kidney function was 11 per cent.
Dave Stanley, 36, stepped in to donate Zac, now eight, the life-saving kidney just in time for Christmas.
Mr Stanley said: “Giving up your kidney or an organ for your child is just what parents do.
“Afterwards, just seeing him sitting there being his cheeky self eating a banana was amazing. I couldn’t ask for any more, it is amazing to see him so well.”
Zac had to stay in Alder Hey children’s hospital in their home town of Liverpool for five months as a toddler, having dialysis for chronic kidney diease caused by the E. coli bacteria.
Nikki Berg, 27, said: “We were told he wouldn’t need the transplant until he was a teenager, but because his kidney function has deteriorated over the years he needed it sooner.
“It was a shock but such a relief when we found out Dave was a match.”