If it's just repertoire for vocal teaching, with piano - vs. arias he'd ever consider auditioning with to sing with full orchestra when his voice is fully developed - I'd forget all about "fach" and focus on finding arias that will work on
what his voice needs most, be it making the middle register bloom (resonance and overtones), upper extension, breath management, falsetto mix, etc.

Obviously, there are extremes to which you wont' go - you're not going give a developing dramatic arias for haute-contre and tenorino - but lyric tenor and and a few spinto arias will probably get you want you need. Think along the lines of:

Verdi: "Parmi veder le lagrime" -

- from Rigoletto

Verdi: "De' miei bollenti spiriti" -

- from La Traviata (for upper extension work)

Verdi: "Ah, la paterna mano" -

- from Macbeth

Puccini: "Recondita armonia" -

- from Tosca

Bizet: "La fleur que tu m'avais jetée" -

- from Carmen

Mozart: "Fuor del mar" -

- from Idomeneo (for work on agility and lightness - which all singers SHOULD have, at least to some extent, regardless of fach)

Britten: Tarquinius does not wait -

- from The Rape of Lucretia

Von Weber: Durch die Wälder -

- from Der Freischütz

Tchaikovsky: "Kuda, kuda" -

- from Eugene Onegin