The Arsenal fan was playing with pals when he fell on to the fence – which pierced three inches into his chest.
He managed to pull himself off the spike as friends rushed to find his mum, Jackie McGlade, who found the 10-year-old screaming with the pain.
But she was left stunned when doctors at Sunderland Royal Hospital told her the spike had missed all John's vital organs – because his Gunners' shirt had cushioned the impact.
Jackie, 39, at home in Tunbridge Road, Thorney Close, said John would've died if he hadn't been wearing his shirt.
John, who is fanatical about Thierry Henry, and even shares a birthday with the Frenchman on August 17, had been playing in a friend's back garden when he climbed on to a two-foot wall to look in to the garden next door.
He lost his balance and fell on to metal spiking on top of the adjoining fence, which runs around the flats in Toronto Square.
Single mum Jackie said: "It doesn't bear thinking about. The doctors said he's been very lucky not to have damaged a vital organ. If the spike had gone in any further it would have ripped through his lungs and heart.
"He could've been killed and if he hadn't been wearing his Arsenal strip he probably would have been.
"When I looked at his shirt afterwards I couldn't believe it. It hadn't even been torn, there was just a little dent in it and blood where the spike had gone through, but amazingly it was still in one piece."
John, a pupil at Farringdon Primary School, has been a fan of the Gunners all his life and will be glued to his television this Wednesday with four-year-old brother Liam to watch his team take on Barcelona in the Champions League final – and he'll be wearing his beloved top.

Jackie added: "I was frantic when I found out. I rushed round to see him and he was in so much pain. You could see right through the wound and some of his flesh was hanging out.
"We didn't want to wait for an ambulance so we drove him to hospital. The whole way there he was screaming and crying and saying 'Don't let me die, Mam'. It was so traumatic for him.
"He's mad keen on Arsenal. The first thing he said when he came round from his operation was "Mam, I can't believe my Arsenal strip is ruined'. But of course, it wasn't. It had saved his life."