If you still have contact with the Amish that trained him, I'd ask how they did it.
I had a horse that was a pain to bridle, I used a rope halter as suggested and taught the head down cue ala John Lyons, but that took a couple months to get right. I would cheat, and don't flame me for it, but I would stand on the lead (hooked to the halter) and then try to bridle. The horse knew that tug on the halter meant head down, so it sorta worked...lol... Not the best method and be careful you don't get your feet caught, but it worked for me. Eventually the horse gave in and allowed bridling. It was the least stressful approach and retreat that I could work out...
Wish I had better advice for ya, but I've never had to deal with such a big fella!
Be strong and courageous, and do the work. Don't be afraid or discouraged by the size of the task, for the Lord God, my God, is with you. He will not fail you or forsake you.
1 Chronicles 28:20