At one point I had 15 stallions here, and although they were separated with their mares for breeding season, I ran almost all of them in a bachelor herd after the breeding season had passed. The majority of them came from other places, and I heard horror stories about several of them, but each fall, I put a large "round bale" of hay in a "new" pasture where no one had "territorial rights" from earlier in the season, and then moved in each stallion one by one with me walking them in the "new" paddock to be there to discourage any bad behavior or "sassy" boy behavior. After all were together for a day or so, and eating well and behaving, I moved them to the larger wintering pasture area, and they got along just fine.
I did keep a couple of my "senior" old Falabella gentlemen with their "mares" over the winter, as the Falabella gents like to be with their girls, and they were well mannered with the girls and new foals.
Keep working with him, and I think you'll be very happy with the outcome. Thanks for sharing the progress with us!