Without knowing his age isn't hard to give you an idea of what could be going on. It could be anything from cancer to an infection to something stuck in his nose.
Here is some accurate information to check out:
https://vcahospitals.com/know-your-pet/nose-bleeds-or-epistaxis-in-dogs -- most relevant details "Place an ice pack on the bridge of the nose (on top of the muzzle).
In short-faced breeds, be sure your pet can breathe around the ice pack. The cold will constrict small blood vessels, which will slow the bleeding. If these steps do not stop the bleeding or the pet is having difficulty breathing, see your veterinarian or emergency clinic at once.
Remember that a pet with a bloody nose will likely swallow a great deal of blood. This may lead to a black stool (melena) or vomit that contains blood clots (hematemesis). After an episode of epistaxis, these findings are common and do not necessarily indicate bleeding in the gastrointestinal (GI) tract."
I hope that helps!