Your PRE stallion was trained for a year, by who ? All of the behaviors are dominate/aggressive I first have to question the NEVER geld as these are his natural behaviors they WILL be passed onto any get he sires. Not a trait to be passing along. Next he doesn't need to be desensitized he needs to be put in his place. I may try the bag thing with a lethargic good meaning gelding. You have a stallion-you need to try to "kill " him. The one that got a couple good hits in is right HIT him. If your fast enough to stay out from under him attach a lunge line with stud chain. When he challenges you in anyway stay slightly to the side use the chain and send him backwards fast, always have your whip as well and wear gloves[ If he strikes a helmet wouldn't be a bad idea either] While loungeing don't let him come in that shows him he is above you, teach him whoa you go to him to change direction or pet or for any reason. Yes, its easier to stand still and have him enter YOUR space, problem is he should NEVER enter YOUR space. Him stopping, again he is in charge and just letting you know that NO he is not going to work today, press the issue he turns and makes a stronger statement that he really isn't working today. Try to treat him as a stallion not just a horse and certainly not your friend. Not trying to be abrupt, just don't want anyone getting hurt.