What a bizarre case. OP, I am so sorry you and your horse are going through this.
I have one question: is this horse still being bullied in the field? I ask because that could be exacerbating both the physical and mental components of the issues you’re seeing (including balking when being fed).
What about giving this horse 6-12 months in a field (with shelter, hay/feed as necessary), and an older buddy or 2 who won’t bully him … so long as he is comfortable in turnout (and it sounds like he ism bullying aside).
Maybe you could find a retirement place nearby that would be cheaper than where he is now.
This would lessen the pressure on you to make any decisions, ease the financial burden, allow time for the experts, and maybe give him to heal…if he can.
Good luck, OP.