He's sixteen. Eighteen is when one becomes a full legal adult, but there's a gradation of responsibilities available before that. A child can apply for a trainer's license as young as ten. As old as he is, and having lived more or less independently for so long already...they may just allow him to become emancipated. That would mean he was granted legal responsibility for himself in spite of his age.

[It's probably the best route; it would mean more or less continuing his life as normal, without any disruptions from long-lost mothers. Perhaps he could be a ward of the school or something too.]

He will be provided for financially, at least. The Professor did have a will, and he was actually fairly independently wealthy after the Tera orb did so well. [You wouldn't know it the way he was living, but]