Prosecutors believe Alex, who is now 17, has been found in Revel near Toulouse, southern France.
Local reports said he fled a rural spiritual community in the Pyrenees mountains and hiked for days to escape.
His grandmother Susan, then 62, thought they might have adopted "an alternative lifestyle" somewhere abroad.
She said at the time: "They didn't want [Alex] to go to school, they don't believe in mainstream school."
Alex was expected to leave Marbella after just a week in the town of Benahavis, but he and his family never returned.
They were expected home on October 8, but inquiries at the time suggested they may have sailed that day from Malaga to Mellilla on the Moroccan border.
Cops launched an investigation into his disappearance, but until today he remained missing.