It sounds like the beginning of a respiratory disease. Isolate him immediately, as respiratory diseases spread quickly and can be serious. As to the exact respiratory disease that he has, it is difficult to tell. It could be a viral disease, or a bacterial one.
I'd isolate him, and start giving him some electrolytes and probiotics. Keep him out of drafts and monitor him closely. Make sure that he eats enough. If he isn't eating enough, tempt him into eating with some scrambled eggs, moistened chicken feed, or applesauce. You can also give him some raw egg yolk, and yogurt.
If you don't see any improvement, or if he gets worse, it is probably a bacterial disease like Chronic Respiratory Disease. If you want to treat the whole flock, or if you don't feel comfortable giving injections, you can start out with a water soluable antibiotic like Duramycin, Terramycin, or Tetroxy HCA-280. However, a better antibiotic would be Tylan50.
Tylan50 is given as an injection generally, but can be given orally as well. Injecting is better, as it treats the bird quicker. Give 1cc for large fowl chickens or large birds, and .5ccs for bantams or small birds. You'll need some small needles (20-22 gauge) and some syringes. Inject it into the breast muscle, switching breast sides daily to prevent too much irritation. Give Tylan50 once a day for five days. If its working, you should see improvement by the third day.
Keep in mind that many respiratory diseases make birds carriers. This means that they can spread it to other birds, even if they don't show the symptoms. They may show the symptoms during signs of stress. Of course, sometimes birds that recover don't ever show the symptoms again. It all depends on how stressful their life is, and how resistant they are to the disease.
It is possible that this is not a respiratory disease. When my roosters crow, they often draw in their breath quickly, and make a sort of squeaky sound some of the time. Sometimes, it sounds like a sort of moan.
Hope this helps!