Stop pup from biting older dog.?

We have a male yellow lab-3 months old. We've had him about a month. We also have a 2 yr. old er spaniel, usually a very mellow lovable dog. In the past couple weeks they have started to bite and growl at each other pretty aggressively, enough to make each other yelp sometimes. are they just playing or are they fighting & what do I do to break it up, haven't been successful with "no" so far, I have seperated them. The vet t. I asked about it said I shouldn't let them fight cuz the pup may tear the er's ear or scratch his eye (which sorta bulge out a bit) with his sharp teeth.