Liam Wooldridge spent the money playing Small Worlds, which invites users to create their own virtual universes.
It is free to join but there are options to pay for extras, such as VIP membership, which grants exclusive access to parts of the site, as well as credits to buy items within the game.
Kay Flanders is warning parents after son Liam Wooldridge racked up a huge bill on games
Liam’s parents were shocked to see his usual £10-a-month phone bill rocket up to £850.
The 11-year-old had also signed up as a member until 2020.
The debt has now increased to nearly £1,000 with added admin costs as solicitors on behalf of the phone’s service provider T-Mobile have sent out letters warning bailiffs may be involved.
His mother Kay Flanders, of Plover Road, Minster, said her son played the game on a laptop but all the purchases were made and confirmed through his mobile, which is registered in her name.
The 49-year-old, who is a beauty therapist and also works on the switchboard at Sheppey Community Hospital, said: “It was like he was playing with toy money not real money.
Liam Wooldridge was playing on the game Small Worlds
“He just pressed the button to keep buying these things. He hasn’t received anything. It’s just bits to this game on Facebook. It’s just on screen. It’s a complete madness.
“How can they do this on a kid’s game? No sane adult would be playing this.
“Liam has been so upset. He’s offered to sell his Xbox, his TV, everything to pay for it.”
She added the money was meant to go on a break away for the family which they will not be able to go on if they have to pay the bill.
“How can they do this on a kid’s game? No sane adult would be playing this" - Kay Flanders
Ms Flanders said she has written to the company and the game’s developers hoping to get a refund on some of the money but neither have responded to her.
T-Mobile says it is looking into the bill but did not comment further.
Small Worlds has not responded to our request for comment.
A spokesman for PhonepayPlus, which regulates phone-paid services in the UK, said it would need to understand the circumstances in more detail but in some cases it has powers to ensure consumers receive refunds.