It is dominant behavior, he's basically telling you to "Hand over the grain and f*** off." But wailing on him with a whip is the wrong thing to do. You need to assert yourself as alpha at feeding time but in a way that he understands AND respects. So take a whip in with you and have him stay away from you. If he comes in without permission drive him away. You don't have to yell or be aggressive, just be ASSERTIVE. A lunge whip would be best, that way you have a longer reach if you need it. DO NOT let him in until he has a nice look on his face and he "gets the idea." Then allow him to come in and if he shows ANY signs of dominance (pinning his ears, lifting a leg, etc) drive him away IMMEDIATELY.