My great dane puppy of 8 months old has bad diarrhea, please help.?

Our dog has had Diarrhea for the past three days. He seems fine and eats and drinks. He is still up and about. But it is sometimes slimy, and sort of red, but not a lot. And its like he can't hold it in! I really need someone to give me some advice. We are calling the vet tomorrow But i would like some opinions. I'm just really scared cause great danes come pre-loaded with a not so hefty immune system.