Siddak Singh Jhamat, known as Sid, was given a fossil hunting kit for Christmas but was actually looking for worms in his Walsall garden when he hit a rock which looked like a goat or ram's horn.
"I was trying to maybe dig for worms and pottery," Sid explained.
"I found a couple of worms, not very much pottery. I found, as I call it, Victorian bricks but then I thought I'd dig a bit deeper and I found the fossil.
At first he didn't realise the significance because it had mud on it and thought it could have been a horn or claw. But once it was clean Sid was sure it was a fossil - even if his father wasn't yet convinced.
WATCH: Sid shows where he found the fossil
"When dad washed it, I was like 'this has to be a fossil, I don't care what type of fossil it is, we're sending it to the museum'".
His father Vish said Sid, who attends nearby Park Hall Infants, had come running into the house excited at the discovery.
He said: "I thought it belonged to a goat or ram as it looked kind of conical-shaped, but Siddak is absolutely fascinated with fossils and said it definitely was a fossil, which kids do as they are very self-assured.
"I went onto the UK fossils Facebook page and showed a picture of it, which the advanced enthusiasts on there picked up on very quickly and said it was a Rugosa coral, something from the Paleozoic Era.
"We went digging again in the back garden and found another piece which resembled that, then two days later, he went digging again and found a whole block of corals, including crustacean shells and molluscs."
The Paleozoic Era started 541 million years ago and ended around 252 million years ago.
Mr Singh said he had taken Siddak on trips to sites at Wrens Nest to dig for fossils and down to the Jurassic Coast in Dorset, where hundreds of fossils are buried, in the past.
But he had never imagined that the most exciting discovery would be in his back garden in Greenslade Road could have been a place full of fossils.
He added: "This area used to be part of Manor Park in Walsall and was just green land before it was bought by developers in the 1950s.
"It never looked like the sort of place you'd find something so old under the ground and Siddak was super chuffed when he found the fossil, running into the house covered in mud and smiling.
"He wants to start up a museum in our garage, but I think the plan is to collect more and then give them to a natural history museum, with his name next to it to show when and where he found it.
"I've gone from annoyed at him digging up my garden to being very proud of him for the discovery he's made and I do hope it can inspire people to go out and look for fossils themselves."