That's a real issue! I had an Appy gelding who went totally ballistic when his turnout partners were changed. Every time! I had him for 3 years, and nothing we did made things better. We turned him out with our mare, and he attacked my daughter when she tried to take the mare out of the paddock. We put him out alone in a paddock adjoining another solitary male, which helped to a point, but not everyone has that option. He was 9 when I got him, I had no history on him, and I always suspected he might have been cryptorchid or late-gelded, but never had the proof. When all else failed, we turned him out with the broodmares. He ran another gelding through a fence to "protect" what he considered to be "his" mares. 
Nice horse, and I rode him and showed him, but that separation issue was with him as long as I knew him.
That said, you might want to try Command Serene. It's a B-vitamin supplement that sometimes help calm upset horses. You feed it daily. It worked for my mini stud after I had him gelded. He was still hormonal and VERY dangerous to the other horses, so I put him on that supplement. It took some time and some ground work along with it, but he's a total charmer now.