I know what you mean by jerk roosters!
I used to volunteer at a wildlife rescue & the first thing they'd do with every bird, sick or injured, was warm them--funny I can't remember if they used heat lamps or heating pads. Either way, you need to have part of the box or cage heated & part not, so he can find the right temperature for himself. For sure get him away from the other chickens in case it's contagious, & also you don't want to get cannibalism going in your flock. One other thing they taught me at the wildlife rescue was that by the time a bird really looks sick, it's usually pretty serious. In case you never heard this (sorry if you have!) since birds are prey animals they hide symptoms of illness for as long as they're able or the predators will know they're an easy meal. So he may have been sick for quite a while.
I don't have a lot of other suggestions, but if a sick chicken isn't eating or is thin my vet recommended canned cat food. Also red foods seem to appeal to chickens. I had one get sick & all she'd eat more than a couple bites of was cherry cheesecake! (Well, at least it had lots of calories! ;) )
So, I guess all I can really offer you is kind of the basic general care for sick or injured birds, but maybe if you provide a little more info somebody who is good at diagnosis & treatment can help you. Some helpful info might include the bird's age, is he eating or drinking, what do his droppings look like, can you hear any breathing sounds, is his "bottom" dirty, anything coming out of eyes or nostrils, or any other observations.
A thought just entered my head-if he is limping & you are seeing his brother attack him, he may well be injured. Is his brother older? One thing I just learned about roosters is that they can take a year or so to mature to the full "macho jerk" stage. That's when my last bantie rooster started attacking me & the guy at the feed store explained it. So, if the age of either rooster is around one-ish, they may have started fighting even though they had previously got along OK. On the bright side, if he's just injured you can still eat him!
Sorry I haven't been much help, but as you hadn't had any other responses yet I thought I'd mention these things at least--things can go downhill with birds pretty fast. Good luck!