Truthfully, we aren't going to be able to tell you anything you want to hear, because over the internet, not one of us can see what is going on. And even then, without an exam, it's still not something we'd be able to diagnose. It could be any of the things posted above, it could be that he has a pinched nerve in his back or shoulder, something could be out of alignment, it could be a thousand things. I definitely would get him out of the pasture full of horses that don't like him. Their running him and keeping him an outcast is certainly not good for his health.
But typically speaking, a horse that has a sudden behavior change, loss of muscle, and sudden lack of energy, or refusal to do things he has always done in the past is in some kind of PAIN, which needs treatment. If you can't get a vet to look at him, is there at least maybe a knowledgeable horse person at your barn that could actually put their hands on him and see if he has any pain points? It might give you a place to start.
I would also have a talk with your parents at this point, and explain that the horse acts like he is in pain, and that he needs at the very least, a quick once-over. I assume that the horse is up to date on shots and things, when will the vet make another scheduled visit to your barn? If he is going to be due for the basics soon, it wouldn't be a big thing to look at him a little more in depth then.
"Keep a leg on each side and your mind in the middle"