Alex Batty's mum was involved in a cult and allowed him no contact with the real world, prosecutors have claimed.
Alex, who is now 17, disappeared with his mother Melanie and grandad David in 2017 but is expected to return to the UK in the next few days.
He had gone on a family holiday to Spain and is believed to have been abducted by his 43-year-old mother to live an "alternative" lifestyle abroad.
The teenager was found near the French city of Toulouse on Wednesday and taken to a police station in the village of Revel by chiropody student Fabien Accidini.
French prosecutors claim Alex knew his alternative life with his mum "had to stop" after she planned to move to Finland.
Toulouse assistant public prosecutor Antoine Leroy said Alex left the home and went walking for four days, covering around 15 miles by foot and skateboard.
He was sleeping in the day and walking at night, fuelled by eating "whatever he found in the fields and gardens".
Mr Leroy added: "It is possible that the mother has gone to Finland as she planned.
"The grandfather, who has always been with his daughter and grandson, is said to have died around six months ago."
Describing Alex's lifestyle over the past six years, Mr Leroy said: "They would work on the ego, there was meditation work — there was no connection with the real world. They believed in reincarnation."
Mr Leroy said Alex was tired but in good health, adding: "He's said to be intelligent even though he's never been to school in this period.
"He doesn't describe any kind of physical violence, without talking about emotional violence.
"We can't use the term 'sect' as such but he talks of a spiritual community."
Mr Leroy suggested the group were fascinated by solar panels, and that Melanie has a fear of them.
He added: "They were travelling from house to house with solar panels. They only used car-sharing, they didn't have their own vehicle."
Mr Leroy added: "What I'm describing is what happened in Morocco, in Spain and in France — always the same way of living."
Alex left the UK on September 30, 2017, and was due to return on October 8.
Stephen Devine, 42, a cousin of Melanie, said yesterday: "Alex's mum was involved in a cult.
"His gran became his guardian but then Melanie and his grandfather offered to take him on holiday for a week and were never seen again.
"It is going to be a big adjustment for him. He's probably grown up without a formal education."
It comes after the shack where Alex lived with his mum and grandad was revealed.