What ever has been brewing in your little guy has been going on a lot longer than you may have noticed. There is a pecking order among most species, and in order to hold his "rung on the pecking order ladder" an individual in a group will hide his symptoms of illness from the other members (and from predators) until the illness is deeply set in. Therefore, since he was hiding his symptoms you may not have noticed, and it is no fault of yours.
He could have parasites, viral, or bacterial infections, and more, and as ireno has said it may be too late, but as peekawho has said, he deserves a chance so take him to he vet.
He needs to go to a vet right away.
I fear it may be too late. When an animal becomes that lethargic and non-responsive it usually means the end is near. Peek is right - if your little rat is still alive, he needs a fighting chance and a vet can help by doing subQ fluids and testing for illnesses. If he doesn't have bite wounds then I doubt the other males did him any harm. I'm guessing he got sick somehow or has a blockage (the hunched posture).