• The 17-year-old said he was constantly on the move with his grandfather and mother after they disappeared in 2017, and wanted to get his UK life back.
• The officials said he should be able to return to his family in the UK on Saturday or Sunday at the latest.
British teen Alex Batty, found after six years missing, decided to leave his mother because she wanted to move to Finland, French officials said.
The 17-year-old said he was constantly on the move with his grandfather and mother after they disappeared in 2017, and wanted to get his UK life back.
Alex was found by a motorist in southern France on Thursday.
The officials said he should be able to return to his family in the UK on Saturday or Sunday at the latest.
The current whereabouts of Alex's mother - who does not have parental guardianship of him - are unknown.
Alex himself has not said where she is. They lived in Morocco for two years and then in the French Pyrenees but he has not said exactly where.
His grandfather is believed to have died six months ago.
Greater Manchester Police (GMP) said his Oldham-based relatives were "massively relieved" and going through a "whole host of emotions".
In a statement issued through GMP on Friday afternoon, Alex's grandmother Susan Caruana, who is also his legal guardian, said: "I cannot begin to express my relief and happiness that Alex has been found safe and well.
"I spoke with him last night and it was so good to hear his voice and see his face again. I can't wait to see him when we're reunited."
Requesting privacy for the family, she added: "The main thing is that he's safe, after what would be an overwhelming experience for anyone, not least a child."
Toulouse Assistant Public Prosecutor Antoine Leroy told journalists the French authorities were still trying to piece together Alex's life since he went missing.
He said that the teenager came across as a very intelligent boy who was very easy engage with, despite having no formal education since his disappearance.
Alex told French officials how the three of them were always moving around as part of a small community of about 10 people, Mr Leroy said.
The area in the foothills of the Pyrenees is known for attracting people in search of alternative lifestyles.
It appears the group had a phobia for certain types of energy and took solar panels wherever they went. He says that at no time during his disappearance was he enrolled at a school.
But he spoke about the community as being "spiritual" and the word cult was not mentioned, Mr Leroy added. He said he suffered no physical violence.
The assistant prosecutor said that when Finland was mentioned, the teenager decided to "get his life back in the UK".
"When his mother indicated that she was going to leave with him to Finland, this young man understood that this had to stop," he said.
"So he then decided to leave the place where he was with his mother and he went walking for four days and four nights."
He walked during the night and slept in the day, getting food from fields and gardens, Mr Leroy added, until he was offered a lift by delivery driver Fabien Accidini.
Mr Accidini spoke earlier about their encounter, saying that they talked for more than three hours in which Alex told him his story.
The delivery driver said he lent the teenager his phone and let him use his Facebook account to contact his grandmother.
His first words to his grandmother for six years were: "Hello Grandma, it's me Alex. I'm in France Toulouse. I really hope that you receive this message. I love you, I want to come home."