So what is he doing that is aggressive? Not that it really matters but some acts of aggression are more dangerous then others. You handle the aggressive behavior by teaching him you are in charge and delivering quick consequences for his bad behavior. You have to show him you can control his feet and be able to move his move feet left, right, forwards and backwards. That respect comes from doing lots of ground work and teaching him you are alpha and demand respect. To keep him corrected you have to be consistent, you cannot let him get away with it one then correct him the next day for the same behavior. Do not be afraid you will hurt him do you think the alpha mare cares if she hurts a disrespectful herd member, not a bit. Within reason you are not going to do anything that will hurt him.
If he gets into your space drive him out of your space, do not allow him to try and dominate you. If he is aggressive at feeding time carry a whip and if he gets pushy whack him with it and drive him out of your space and make him wait at a respectful distance. If he turns his butt to you chase him away and make him hustle his feet. Set him up for success and reward the smallest try, show him you are worthy of his respect.