Hygiene, then wait and see if it gets plugged again. Scrub both sides and clean the openings with a toothbrush. If you need to scrape off the dried dirt and mucous, use a toothpick. Wash with an eyedropper to get all the dirt and mucous out.
Any new chickens join your flock lately? There are a few possibilities. It could be something viral, or could be bacterial.
Any viral cause can't be treated with an antibiotic, but the virus will go dormant after a little while and he should recover and live possibly without ever showing symptoms again.
As for if the cause is bacterial, tylosin (tylan) can help common bronchial infections. It is an antibiotic.
VetRx I have never used as I live outise the US. Acv is good for if you think he may have a worm infection contibuting to his immune system weakening. If you want to give him medicine for a possible bacterial infection, that would be an antibiotic like Tylan. If you think your flock is at risk of worms, maybe treat that, they can cause a weak immune system. Otherwise you need to just clean his nostrils and beak and see how he does for a while.