As I recall he's trapped in the Library of Dusk. You need to rescue him in order to find him at the Consultant's House in the Hollow city after it. Magical knowledge is different. Telenger is a respected scientist mage, a researcher and an explorer, not a hands-on mage in spite of his attempts to be the one.
You mean one of those quests related to him in the Aldmeri Dominion? Yeah, I haven't done it yet. I don't like the Dominion so that is why I'm a bit slow with progressing in it's territories. Has he prooved himself to be also a skilled battlemage there?
Ah, finally I think I've understood what @Dusk_Coven asked. I thought he had meant some side quest in one of the Alliance regions not the main one while mentioning my quote. Thanks, @RaddlemanNumber7.
Telenger has been probably been assigned to the Mages Guild equivalent of latrine duty for the duration of his stay in Coldharbour.
Yes I heard that conversation in Stirk. But it's the end of the world. Vanus is really that petty? o.O
I'd have expected the Vanos siblings to be swapped out for Telenger.