It could be that he's caught a cold or its irritated by something, or he could even have allergies. Some breeds - the ones with pushed in faces- tend to have runny noses more often than breeds with normal faces. My pug has a runny nose often (I'm forever wiping it) and I've had him checked and its just his puggy nature, he's in perfect health. If its something new and continues for several days or your dogs has a cough or any other symptoms I would have him checked by a vet just to be safe.
My dog had the same issue we took him to the vet and the vet said he was healthy except for the fact he was 3 pounds over weight they charged my dad 30 bucks for his weight gain. She said its common in a lot of out door dogs and hound dogs.
my dog is a crocker spaniel. would you call that a pushed in face?