he should have asked you to bring him some from the troll king's mine. he'll also provide you with a special bag to keep it in during transport.
After you return he'll teach you how to handle it.
Not sure why you wouldn't have already prompted this conversation with him, have you tried all conversation options?
When you refuse his "shady" offer, he never again offers it actually. So you are locked out of tenebrium weapons for 3 of 4 characters, as only -1- skill book with this skill exists, and it's in Attenboroughs office (in the hidden imacculate village, she's the geomancer there). I assume this is the "fall back" Larian added to make sure SOMEONE could handle this in your group.
Kinda weird you can lock yourself out of such a major quest in this way though...