The problem with treating chickens is that they hide their unwellness so well and pretend to be healthy for such a long time, that when we finally notice that something is off it is often too late.
Keep a close eye on your remaining birds to try and catch it early in case it has spread already.
:hugs
You could send him to your state vet lab for necropsy to know exactly what it was.
With the results you might be able to prevent similar occurances in your flock.
There are several respiratory illnesses that can cause what you describe, but the following two are the most common:
MG Mycoplasma gallisepticum Infection in Poultry - Poultry - Merck Veterinary Manual...
Does he have a foul odor about his head?
What you describe could be Coryza which will be treated with antibiotics and a manual removal of the accumulated pus.
https://www.backyardchickens.com/threads/coryza-treatment.1595853/post-27125553
Welcome to BYC! :frow
I am sorry your rooster is sick, please post some pictures so we can better see what the problem is and how to help you help him.