The disclosure that you recently had another chicken die is alarming. It would seem you have a possibly contagious disease in your flock. This fellow is definitely sick, and you may lose him soon. You can try antibiotics, but there are viruses that wouldn't be affected by that treatment. The best and most certain method of diagnosing what is making these chickens sick is to have a necropsy done when/if this one dies.
It's smart to get proactive on this and call your local university extension and ask for the address and phone number of an agricultural lab that will do this test. They can also advise you on how to handle and ship the body or where to take it. This test only takes a short amount of time and you can have results often the same day. The test is not expensive. But the body needs to be handled properly so you will need to know what's required.
You can try to treat your rooster, give him a round of a broad spectrum antibiotic, Poultry Nutri-drench to increase his nutritional levels, and hope he gets better. If he doesn't, then you know you are probably dealing with a virus. If you have an avian vet, it would be a good idea to have this rooster looked at.