If you pick up the tail and it falls back down, that's a pretty good indication it's broken. Not sure what you'd do about it. Or it could just be bruised, or he could be sick.
What stores do you have local to you? While giving him a bath is an option, it's not a terribly effective one as it...
That's lice, but it doesn't usually cause a chicken to act odd. (listless, maybe) I'd treat him and all of the birds he ranges with. Liquid permethrin. You may be able to spray it over them while they roost, if you're unable to catch them.
I've had a thought--is there any possibility he's...
Nah. A chicken with his tail down is sick. A sick chicken is a disease carrier and a danger to the health of the flock. Thus, he's a target (especially in more wild populations. Barnyard flocks are often a little more tolerant... but not much.) Cockerels disguise themselves by not crowing, and...
Sounds like he was having trouble getting oxygen. Can you get him to a vet? If not, please post poop pictures and check the back of his throat to see if you can see anything white (gapeworms.) Is there any possibility he could have gotten into poison?
EDT: Actually, is there any possibility...