Please take him to the vet. If he is left out there unsupervised you have no idea what kind of damage he could have done to himself in the past 6 months, including trachea damage. Make sure tell the vet that he is kept outside on a chain/line, besides trachea damage, he could be suffering from an upper respiratory infection or some other type of infection and could be more sick than he is letting on (animals in general do not start showing problems until they are already quite ill, especially one that is kept outdoors).
Leaving a dog tied up where you know there are loose dogs running around is just an accident waiting to happen. A loose dog could come up and attack your dog who is pretty much defenseless being on a tie out, and he can not run away if a dog is attacking him (several cities across the country are actually starting to ban dogs being left outside on chains because chained dogs have a tendency to become aggressive and also because they are targets for other dogs to attack).
Here is a really good article that could help you learn some things about his current living situation, please read it: Virginia Beach SPCA
I am not trying to be mean, and hope I am coming across as just trying to help your dog and educate you on what could happen to your dog and the potential risks you are taking with your dog by leaving it chained up outside (my intention is to help your dog)... I highly doubt that anyone here would not back me up with saying that leaving a dog tied up outside 24/7 is dangerous (loose in a fenced in yard would be better than tied up).
After 6 months of being outside he is now starting to show signs of a problem... Please, Please take him to the vet to be checked out. I don't think this is a self-medicate or wait it out type of situation, at least call the vet explain his living situation and symptoms and let them tell you if you need to take him in or not.