I'll see if I can get a picture of him for you.
He is about 2 years old.
I have temporarily been feeding them the Purina layer crumbles (I believe it's the 16% protein kind), because my feed store didn't have the Kalmbach crumbles.
Yes, I have checked for injuries. I did notice that his comb was bloody (it looks like he got into a fight with another rooster). I cleaned it.
Yes, his crop has been emptying normally at night.
Possibly. Although, I do check my feed for mold, and make sure my chickens don't get anything that's toxic to them.
I have my chickens in a spacious chicken tractor that is insulated. I have about 40 hens and now I have 4 roosters.
I did see a few mites on him. I treated him for that.
Can you post photos of the housing, inside and out?
They are all in a tractor, do they come out for sunshine/to roam around?
I agree, I'd look at what the roosters may be consuming, if this is only affecting them.
The current one seems to have been in a fight with the others. Even with 40hens, some roosters can be quite territorial. Could the others have been injured from fighting or perhaps became weak by being kept from food/water.
You are seeing Mites instead of lice correct? I'd make sure to dust the sick one very well with a Permethrin based poultry dust. It would be wise to treat all the birds too.
If you are dealing with Mites, then the housing needs to be treated, this is important since the Mites can infest the housing and even if you are able to kill what you find on the birds, the birds will quickly become reinfested unless the Mites are eliminated from the coop/housing.
A permethrin based spray is usually easy to source. For large areas like housing, I'd buy the concentrate, then mix in a sprayer according to directions on the label. Spray housing in 5-7day intervals for several weeks to begin eradicating the Mites. Be sure to get into nooks/crannies of the housing, the nesting boxes, roosting bars, etc. Mites can be hard to get rid of, so you may find you need to treat the housing often.