Air sac mites is something that can cause breathing problems in many types of poultry, especially pet birds, but also in chickens. Here is some reading about them:
https://www.merckvetmanual.com/poultry/air-sac-mite/air-sac-mite-in-poultry
https://birdpalproducts.com/blogs/news/air-sac-mites-blog
I haven’t used Elector PSP, but as long as you used it per the directions, it should not have affected him. But it is hard to know what is going on with him for sure. Mold and ammonia fumes can affect the breathing of poultry and can cause damage.
I agree with Wyorp Rock. If there is any reason to think that he might be ill or a carrier, I would return both birds. I decided a long time ago, that I would not buy chickens from other people due to many possible diseases that can be brought into an existing flock, affecting the others for...