While it can be a sign of respiratory infection, it can also be from nasal plugging in snakes that are about to shed or have poorly shed the skin in or around their nares (nasal openings) from prior sheds.

Watch him closely for signs/symptoms of respiratory infection, and turn up the temperature a bit (a just in case it is a respiratory infection). Double check your husbandry to make sure that it is spot on. Look for early signs that he may be going into shed (e.g. pink belly, darker coloration), and if he does appear to be going into shed, make sure he has plenty of humidity so that he will have a good shed and clear the skin plugging his nares.