Join Date: Aug 2011
Location: Whitehorse, Yukon
Try holding the food directly in front of his nose - pretend he is a baby puppy again - bring the food over his head until his butt drops. DO NOT FEED IF HE BACKS UP! Just get him back to the same spot and try again - no correction!
Be proactive, so if he starts to try to back up lure him forward exactly where he just came from and try again. You may have to do this several times, with him getting nothing before he figures out he needs to sit in the spot that you want.
If for whatever reason you still have no success, try luring him with his back facing a corner of the room and his front facing you. Again, lure him into the sit with him sitting in front of you, right into the corner, so you actually BLOCK him from backing up. Mark and reward, then get him to try again. After a few reps of this, try bringing him out from the wall and let him try again.
Once you can successfully lure him into a sit on the spot and he does not back up then I would move to some shaping games, where you get him to offer you the sits and you only pay for the ones that he does without backing up first.