Fungus infection. Go to any pharmacy and get some Nystatin or Lotrimin. Apply it to his nose twice day, sparingly, and rub it in well so he won't get any of it by licking it. If it doesn't start to look better in a week, or if it gets worse in the meantime, stop the Nystatin/Lotrimin and switch to a triple antibiotic cream, applied the same way.
It can be hard to distinguish between a fungus and a bacterial infection. But we always start with treatment for a fungus for two reasons. One, that's usually what it is. And two, treating a fungus with an antibiotic can sometimes make it worse, while treating a bacterial infection with an antifungal medicine won't do anything to it, one way or the other. So if the fungus medicine hasn't helped after several days, it's probably bacterial, and needs an antibiotic.
No need for a vet visit unless 1) it gets worse, 2) a secondary infection develops (you're probably see a weeping area with crusts), or 3) it doesn't improve with either of the treatments above.
