I run a small rescue. We have a 24 year old TB gelding in the rescue. He arrived in December and settled in just fine. In April we moved to a new farm, that was fine.
Then two of the mares Justin was turned out with left and he had a total melt down. He became very attached to Amber Rose, attached to the point where if he was not within 10 feet of her he would lose his mind and become dangerous due to how focused he was on getting to her.
Amber Rose moved to a new home and Justin ran the fence line for 2 days. He then became fixated with a new gelding. We tried putting him in with Glider but he pinned him in a corner all day and would not let him even move enough to get to the trough to drink. So we put Justin in an adjoining field. He now paces up and down the fence all day. If I bring Glider in while Justin is still out then Justin starts running and screaming. If I bring Justin in while Glider is out then he will just pace his stall and scream. He will only eat if he and Glider are both eating in their fields and he can see Glider or if they are in adjoining stalls in the barn.
I am running out of ideas for what to do. We used to be able to ride Justin but now if you attempt to take him to the ring he spins in circles and screams for Glider (even when Glider is in sight).
Please don't take this the wrong way, but my first thought is to consult a good local pro trainer. In person they can see so much more than anyone can see/infer from a paragraph description online. If sending him off for some retraining isn't in the budget, see if you can do some one-hour sessions here and there as budget & time permits. At least get the pro's assessment.
I would keep him busy. Get him to build more of a relationship with people, with the hope that slowly he'll eventually start to get some of that sense of security with his rider. If you're not up to riding him away from the farm, ride him right next to view of his buddy. If he's too much of a handful to ride at all, do ground-based training. Keep him focused on you. I don't mean that in the aggressive 'keep him running until exhausted' sense. I mean that the handler needs to keep a good flow of requests/behavior/reinforcer going so that his attention drifts to the other field less and less. You can use any training method, as long as it's not something that's increasing fear or eroding human-animal bond. Try to work with him every day. Very slowly challenge him to things he's unsure of, with the goal being of building up his confidence. He sounds very insecure (unable to function without his buddy in sight). You could also try working up to training him out of sight of his buddy by starting the first session right next to his buddy, then moving a few feet away each time, keeping it just below the threshold point of where he gets upset.
As far as the issue with herdmates: who is he in with right now? Am I correct in assuming he's in a field alone, with a shared fenceline with his friend? Can you put him in with someone who can stand his/her ground without bullying him? If he's put with one other horse and it seems like he's bullying the other, sometimes putting him out with a group will help. This way there is no one single most aggressive or most submissive horse.
I suppose it couldn't hurt to try a calming supplement. But there are different products on the market, and very few of them have scientific studies that back up their claims. I suppose it couldn't hurt to try?