It sounds like he has the beginnings of a respiratory disease. I'd isolate him in a dog crate or another area away from the other birds. This is because respiratory diseases can travel fast and far.
Some respiratory diseases are caused by viruses, others by bacteria. A common respiratory disease is Infectious Bronchitis. It is caused by a virus, and is often considered the "common cold" of the chicken world. There isn't a treatment for a viral disease, as they don't respond to antibiotics. The most you can do if a chicken has a viral disease is give it supportive care, which consists of electrolytes, probiotics, nutritious foods (like scrambled egg), and warmth.
However, quite a few antibiotics are caused by bacteria. Chronic Respiratory Disease(CRD) and Coryza are two diseases caused by bacteria. I recommend treating your rooster for a bacterial respiratory disease, as they are just as common as viral ones. You can first try and antibiotic like oxytetracycline (sold under names like Duramycin, Terramycin, and Tetroxy HCA-280). These antibiotics are water soluable, so if more than one of your birds gets sick, it will be easier to treat.
Oxytetracycline is not the strongest antibiotic, though. If you don't see any improvement in your birds, it is either a viral disease, or the antibiotic you are using is too weak. I'd advise getting Tylan50, as it is a strong antibiotic. Tylan50 is usually given as an injection in the the breast muscle once daily for 5 days. You'll need some small 20-22 gauge needles, as well as some syringes. Give 1cc for large fowl, and .5ccs for bantams. If you've never given a chicken an intramuscular injection, here is a link to some information:
http://shilala.homestead.com/injection.html
Hope this helps!