Did you vet take a swab or do any kind of lab work on the discharge? With no odor, I really doubt it's Coryza. Trust me. You'd know it. It's putrid and vile smelling.
If he's having trouble breathing, it sounds like Chronic Respiratory Disease... It could be conjunctivitis, which is most...
Respiratory illnesses in chickens can be tricky. It depends on what they have. Do you see anyone else showing bubbly eyes, discharge, or swelling?
Does he have a sickly smell about his head? Coryza has a distinct odor, and you KNOW it when you smell it.
What antibiotic are you using and what...