Is there a way you can put him in a field alone, but bordering the other horses? So essentially just take his one companion out but he can still see/touch the other horses?
I'd do that and sit in with him for a good day or so. He'll come around, right now you're just the person who comes in and makes him do stuff, but your supposed to be his partner. Spend time with him just fussing with him loose, if he walks away, let him, then go back in a few minutes. Spend time doing something else in his field. I'd personally look into clicker training for him, just because it's so greatly improved my bond with my pony and has fixed my very
fearful mare, turned her into a ride-able horse!
When I got her I could march her through all her ground work exercises - she'd perform them like an expert, but nothing in the world could get her to leave her paddock. Not a bucket of grain, her favorite pony, not being chased out with a stick or being forced to do high speed ground work, lunging for respect nothing
got her to walk out her gate. When we moved I thought the problem would be fixed by the new environment, I was sadly mistaken, it was made worse. Now I couldn't even get her to do her typical ground work exercises, she was even afraid of parts of her own paddock! She wouldn't go to the far side near the gate at all!
After 2 weeks of clicker training I got her walking not just out of her paddock but all around my property and once I have a friend to go with me we'll venture off property with her and the pony.
CT has also seriously changed both of their attitudes towards me, while previously they saw me as "the boss" who came and made them do stuff, now they think they've got me trained! They think "I just have to do ____ and she gives me food! What a silly human". They greet me excitedly, follow me everywhere I go food or not, when they know it's time to train (when I have my cup) they get so
excited, practically dancing in place looking to figure out what new game we're gonna play today.
Of course how you CT is important, you don't want to get a pushy, aggressive horse. You also want to be sure you're reinforcing the appropriate behavior. But I find it's a very fun way to enjoy my horses and have them enjoy me. It's fantastic for mounted work as well. My mare had never been backed before me, with CT I was confident and so was she in knowing exactly what she did right. When she stood calmly for me to lean over her she got a C/T, when she stood calmly for me to get on C/T, when she walked when I squeezed C/T and so on :)
Here are some videos I used to learn clicker training: Video 1
There are 3 videos, getting started, ground work and mounted work. That same woman has a blog on how to teach pretty much any skill you could imagine for a horse to do! It's pretty fantastic. :)
My pony has just learned how to get his halter off the hook and give it to me xD
I really hope some of this will be useful for you :) But to be honest, there are some horses I just won't ever love - I can "love" all horses, but not all will be special. Try it all, give it time, but don't feel bad if you aren't in love. If you love him, but hate his problems, that's just work - but if you don't love him, he's not for you.
ETA: After reading Quiet's post, who must have posted at the same time as me :) I so understand what you're saying about being a "treat machine". Many horses when they first move really miss their friends, they have a lot they need to work out. Imagine you moving homes, you have a new family, new friends, new school, new teachers, only 1 person is the same. You're going to completely ignore that one person when they need something from you, until you've completely sorted out everything that's new, the old you already know how to deal with, the new needs to be focused on. So the horse is overwhelmed with the change, the loss, and the new. At the same time they Need to still be good! NH ground work is a great way to help repair these issues. But it sounds like the OP has already been using a good number of ground work methods that haven't worked. Like with my mare, she behaved perfectly when the situation was ideal, but otherwise seemed to be too overwhelmed with fear to even know I existed. CT really fixed that, it got her attention back on to me and off all the 'scary' things in the world. Yes you are a treat machine, but there are clear skills the horse needs to perform in order to get the treat. My mare was afraid of my car when we walked by it, so she got a C/T whenever she approached me, leaning on the car. Now she'll walk by the car without any fuss. CT is a great way to rebuild the horse's positive association to you, but the rules need to be followed.
My horses never get a treat without performing the skill I ask, the treats are always handed under their chin so they need to back up to get it. This helps reaffirm the aspect of personal space. NO food will ever be given for space invasion or for lack of attention. If she's not in the mood to pay attention- me and the food leave. They quickly learn their attention should be on you. If she's took focused on the food that day we spend time working on 'leave it' where she needs to look away to get the treat. The videos I posted explain is far better than I can :) But I see where you're coming from!!