I hate to disagree but the pasture dominance only works when you are there. Which is not enough, there is a herd pecking order, and there is nothing you can do about that when you are not there, as it happens.
In my experience, you can either leave the horse alone and let him deal with it, or you can move him to a single paddock and field. My gelding is few brain cells and stands still while being bitten. So we moved the herd and put him with the lowest and he still was the lowest and did the same thing. I just moved him to a paddock where he is the only horse, but can see others during the day and now he is flipping out at night when they are not there.
I would move him into a herd with more dominants and if not, then you need to move him or accept that he just does not get along.