I think I've done the same thing. I don't actually recall ever taking to Brandon, I only sought him out once I read up on Tenebrium - and he didn't give me the option.
I've tried talking to him several times since, and he won't give the option. I've also tried (at someone's suggestion) killing the Troll King and returning to him, but that also doesn't work.
I have the Tenebrium book on one character, but this is rather frustrating! As far as I know there is no way to get around it, and the same applies to Ben's quest.
Last edited by Brexan; 12/07/14 09:34 AM.