Set the date for him. Tell him that if he doesn't get back to you by a certain date, ie the end of next week if you can wait that long, you will then have to set the date and hope he can make it.
I would stress to his assistant the importance of his returning your call as your deadlines are pending.
That should hopefully see some action. Oh, and get your boss onside with setting the date on the chairperson too.
Jackie, Peer Moderator
www.iqps.org