Patrick McAlindon, aged seven, found the grenade remains when he was out testing his new kit on December 25 during a family holiday to Aviemore, Scottish Highlands.
After just 15 minutes of scanning the beach at Loch Morlich, Patrick was stunned when his new gadget showed an "off the scale" reading.
Patrick discovered discarded crates, fragments of grenades and a bomb warning sign left behind from allied forces more than 70 years ago.
Treasure hunter Patrick, from Kilmacolm, Inverclyde, said: "The metal detector almost went off the scale.
"I was really shocked, I thought it was gold or something.
"We started digging then two minutes later we found the sign.
"I was really surprised and shocked when I saw 'bomb'.
"I was a bit scared at first because I thought there were grenades under there."
After finding the historic sign, understood to date from the Second World War when Allied Norwegian soldiers trained in the area, the family phoned the police.
The dig attracted crowds of tourists as the police called bomb disposal experts.
But after being told there was no risk posed to them, the family kept on combing the beach.
Mum Ana said: "When we started digging we first saw lots of bits of glass under the sand.
"It seemed odd to see so much clear glass bits of bottles at such depth under the sand.
"Then we found metal hinges from the crates, then we saw the metal sign.
"We googled the sign to see what it was and realised that sign was inside a Granade crate.
"After reading that I said 'right everybody, step back'.
"I grew up in Croatia during the 90s, so I knew all about the danger.
"Andrew called the police and googled 'A W BOMBS' and the same metal sign was on a crate of grenades.
"The beach was buzzing with people coming off coaches and playing rugby.
"We sent photos to the police and they sent them onto the bomb disposal squad, who said it was safe.
"Although I was pretty apprehensive they told us to just keep digging."
Patrick, a pupil at Kilmacolm Primary School, is looking forward to telling his friends and schoolteacher of his find when he returns after the holidays.
Ana added: "Patrick loves history. He doesn't watch cartoons anymore, just the Discovery Channel and history documentaries.
"He wants to find gold next."