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?