It's possible he could have a respiratory infection which is treatable with antibiotics. In past years I had a rooster or two develop a pathetic crow, and we treated with antibiotics, they recovered with no issues. Anytime I have a sick bird, I confine it to a pen with heat, where it also has some privacy, and do nothing for the first 12 - 24 hours besides provide feed and water and just let the bird rest, calm down, and get acclimated to the weird surroundings. I think a lot of handling and confinement in a bird that isn't used to it, can be so stressful and doesn't really help a sick bird. I can evaluate and observe during that time, and then the following day I can decide which treatment I want to pursue.. and examine the bird more closely. I find this is a good system except in cases where of course, it's very obvious what's wrong with the bird, which in most cases, it isn't. You do want to isolate him from the flock, of course, but I try to have my hospital pen away from the flock but in a location where the sick bird can still see and hear the other birds. It isn't always possible, but it's less stressful when they can.
I learned on here recently that mullein is a good treatment for birds with respiratory issues, and you should be able to get it in most health food stores, or it's even sold as wild bird food in some places. . I don't know how much to give, since I have not tried it, but it is something that you can mix in the food if he's eating, and it won't hurt, might help.
Hopefully it isn't bird flu, but if it is you will certainly know pretty soon because it is so contagious to the other birds. Don't panic just yet, there's lot of things that can cause respiratory issues in birds.