Are there any other horse combinations that can be tried? Sometimes geldings can be pretty mean to mares, or vice versa. I had a really nice QH that I raised, and when I moved him to a different facility, he ripped into every gelding there, even when the geldings were separated from the mares. So sometimes these things just happen.
I would see if you can't try different horse buddy combinations. Otherwise as long as he can see another horse, he doesn't 'need' a buddy, especially if he's going to be mean to another horse.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."