I would send him in straight into the blind (not around like a regular search), and have a long check line attached to something that will correct him (lets say a prong). As long as a hard correction on the prong won't shut him down...right before he gets to the helper and tries to get dirty...check him and make his position where you want it to be. You might have to check him farther from the helper and let him get closer the more you do this.
You could also lay down a flat board or something in front of the helper, which allows the dog to put his front feet on it. This way he gets a better picture of where his position should be.
If it helps...start this with the helper between two blinds (close together) so that the dog knows not to cross the threshold made by the two blinds.