"Santa", whose real name is Mr Hussain, was walking down the street in Tooting in December 2013 when he was spotted by Alfie, who is now 6. He heard the little boy call him Santa, and turned round and gave him some money, leading Alfie to believe that he really was Father Christmas.
Hussain, a Muslim man who works at an accountancy firm down the road and just happens to have a big white beard, has since called back at the house every year with a gift for Alfie and his 13-year-old sister Hayley.
And after getting to know the family, he now even calls round on their birthdays.
Alfie's mum, Tracy Ashford-Rose, told BuzzFeed News that Christmas wouldn't be the same without a visit from "Santa".
She said she remembers "my little Alfie shouting out for me, 'look it's Father Christmas!'"
Hussain "got three houses away and came back to knock on my door; he said he couldn't pass without giving a gift as my little boy thought he was Father Christmas," Ashford-Rose continued.
"This was four years ago, and we still exchange gifts to each other. He comes to see Alfie and my daughter Hayley on their birthdays and gives gifts, he never forgets the dates.
"We now call him 'Grandad Christmas'. We see him throughout the year, he shakes Alfie's hand, and has a cuddle. Christmas wouldn't be the same without him now."
Amanda Taylor-Purchase, a professional photographer and the family's former neighbour, was at the house when Hussain called round last Sunday, and she shared the heartwarming story on Facebook.
"It's not very often that I get a tear in my eye at a photoshoot, but that guy just did it for me. I thought I had to tell that story," she told BuzzFeed News.
"I've known Alfie since he was just a baby, there's not much that stops him in his tracks, but when he saw 'Father Christmas' he came running into me and said 'You've got to come, the real Santa Claus is at our door, he comes to our house.'"
Taylor-Purchase added: "When I looked out I was a bit confused," because Hussain does not actually look like Father Christmas.
"Not at all," she said, "obviously it's just the beard association for Alfie – he's a little fellow, very skinny, wearing a business suit. It's the sweetest thing, Alfie doesn't see the colour of skin, the colour of clothing, he sees the colour of someone's heart.
"This guy looked so friendly and so welcoming and he saw his big white beard and thought he was Santa Claus."
Taylor-Purchase added: "One year there were balloons outside and he saw it was his birthday. Now he stops by and gives him a birthday present, and a Christmas present, it's become a tradition.
"For him to come back and knock every year and not forget his birthday, not forget Christmas, and even remember his older sister who doesn't believe in Santa – it's just the sweetest thing."
Taylor-Purchase posted the story on a local Facebook group, and many people were so touched that they said they wanted to give their own gifts to the "real Santa Claus".
Accompanied by Alfie, she took two bags filled with treats down to surprise Hussain on Friday.
"I don't want to make assumptions but if he's not a Christian man, if it's not his faith, but he's still doing all of this for a little boy, he's just going above and beyond," Taylor-Purchase said.
BuzzFeed News tried to reach "Santa" to comment on the story, but his firm was closed on Christmas Eve...