Good luck! I will say this, because your horse sounds so much like how my horse used to be, be prepared to do that exercise for a couple months. I know that sounds like a long time but really it isn't. Some horses you can do that with them for a week and they're fine, but with horses like this they have a lot of issues so it takes consistency to teach them, just like with anything else.
One question for you though........are the muscles on the underside of his neck bulgy? Like over developed?