I had this problem yesterday...
If you hail the seated version of him that means someone else has just completed the quest ahead of you (the correct one is standing, he doesn't sit until the quest is complete). Even though you can force him through the dialog by hitting 'H' to hail him you won't get him to do anything.
What you need to do if you are stuck in that state is to delete the quest and get it fresh from a standing version of him - after that you can hail him again and start the quest.
Though - from the sounds of it - they are fixing it so the above will likely be moot soon. 