I can't imagine Théo leaving his family on his own will. His flight ticket was booked and payed in advance, I presume, for cheaper rates. He seemed so eager to see them and his friends. He could have gone back to Australia after telling his parents he changed his mind or even told them by phone about his new life plan. No way !
Totally agree. Of course possible, but highly improbable here.
The three scenarios are:
1. He’s in the Tallow Beach bush.
2. He’s in the ocean.
3. He’s been transported away.