Hello and welcome to BYC!
Uh oh! So sorry that your roster is sick.

I am not an expert but I have a few questions that could help narrow down the possibilities:
Does he make raspy noises when breathing, cough and/or have nasal discharge (i.e a runny nose)?
How does his comb look (i.e is it pale and/or have any scabs?)
How old is you rooster?
Is he underweight/not eating?
How long has this been going on (weeks or months)?
Does he show signs of external parasites?
Does his plumage look healthy (does he have plenty of shiny, strong feathers)?
Could you post a picture of him?
Hope that your rooster is better soon!
BTW: I would advise giving him some probiotics to help replenish his good gut bacteria and balance his intestinal flora after the use of the antibiotics.
