Apologies if I'm telling you what you already know, but he's in a separate room that you access via a stone door near where you meet Aradin and gang (assuming they didn't leave after the confrontation with Zevlor).
Once you're in the right room you should just be able to walk up to him, or at least I had no problems. If you can see but not reach him, perhaps a screenshot would help?
Also, moving this thread to the Help subforum where it fits better.
