I can see where you would be worried about this “giant horse” getting the upper hand. I ride an off-the-track Standardbred who is 17hh. But he was trained for the track by my friend who is barely 5′ tall!! She is not abusive to her horses, but she definitely doesn’t tolerate aggressiveness. She cannot afford to with a barn full of race horses to train. When I got this horse at 5 years old, he didn’t like me coming in his stall when he was brought in to eat. He wasn’t used to being fussed over–just train/race/rest.
I just went in anyway, and brushed him, and combed his mane and tail, and just spent time in there with him, until he decided I wasn’t going away, and I wasn’t doing anything to upset his dinner. Now, he gets a treat in his grain pan while I groom him in his stall prior to tacking up to ride. It’s our routine, and he accepts it. I even clean his stall around his feet sometimes just to remind him I can.
Routine is what horses love, and you have to set it up for him. When they know what to expect, they are comfortable. They don’t like surprises or changes. Setting up this stall routine is a slow, laborious process, but worth it.
You might also want to consider trying out a calming supplement that would soften his mood and make him more accepting of your presence in his stall. SmartPak has a variety of those available. I use Quiessence with my other horse, a Haflinger gelding that I use for driving that was made into a bully by his previous owners. When I bought him, he was extremely fat from over-feeding and treats, and expected food from people whenever they approached him. This made him a biter, and I would not tolerate that.
It took me several weeks and some negative physical contact to convince him that I was not a “vending machine” or a “chew toy” for him. I like Quiessence because it is a calming supplement and also helps with metabolic imbalances that can cause horses to maintain fat in their bodies (easy keepers). I was not able to get any weight off him no matter how little I fed him, or how much I exercised him!!
It took several months to see a difference, but the supplement really helped him with both of his issues. With diet and exercise and this supplement, I was able to reduce his roundness, increase his energy level for work, and settle his attitude down. I wouldn’t be without it now. He’s a completely different horse now. He’s happy and friendly, and has lots more action on the cart. He even runs and bucks in the pen now–he never did that before!
You may have to resort to someone else with more experience teach your horse some ground manners, and then transfer that authority over to you. I like Clinton Anderson’s approach to ground work. It’s no-nonsense, and he works a horse like a lead mare would do it in a herd. Ask once nicely, tell again firmly, whack! Did you hear me that time? Horses are very intelligent, and learn quickly. It’s just too bad that sometimes they are learning bad behavior, instead of good.
Good luck to you with your big horse. Never let his size make you back down. We are smaller, but we are smarter.