Supposing that he really just gets tired of the feed, and I've seen such horses before (it might happen because the feed is not completely balanced and he feels he doesn't need something in it any more, by the way), molasses shouldn't be the first choice how to brighten up a meal - you're not interested to add more sugars to a horses' feed. Instead, try cold-pressed linseed oil or apple cider vinegar. I know no horse that would turn down apple cider vinegar! Just careful with the vinegar if the horse has ulcers.
I have come a long way, to surrender my shadow to the shadow of my horse.