The source added: "Sadly the victim died from very serious injuries to his head."
At least five other people were in the garden at the time of the accident, including three children aged between 8 and 10.
All were suffering from "psychological trauma", but were otherwise physically unhurt, said the source.
Petanque, one of the most popular games in France, is a form of boules that involves tossing metal balls as close to a small wooden ball as possible.
The main balls are usually made by welding two steel half-spheres together before they are heated to around 900 degrees and then finished.
Investigators are considering whether a manufacturing defect or a chemical reaction may have caused the ball to explode with such horrific consequences.
In July last year there was a similar incident when a petanque ball was left in a fire pit on a beach before exploding and seriously injuring a teenager in the chest.
There are no other known examples of anyone being killed by an exploding petanque ball.