I used to have a gelding that at every feeding time he had a quirky habit of sticking his tongue out the side of his mouth and rubbing the corner of his mouth/lips with it until it was frothy with slobber. It was his own little aniticipation of grain and was harmless. He still does it (my Mom still has him) and it is not a result of any health issues. Some horses paw at their buckets or bang on walls, perhaps your horse is just slobbering in anticipation.
Of course, I have to mention the possibility of a tooth issue or a sticker or something caught in his gums or tongue, but if it's only right before feeding time, it's likely just anticipation, like pavlov's dog experiment.