I agree!! Go to your vet. While waiting to see the vet, let his tummy rest. No food for 24 hours, just water. If there is no more diarrhea , then cook up some white rice and some skinless, boneless chicken breast. Give a small meal of that . 3 parts rice to one part chicken. If your dog continues to have no diarrhea...then try a little more rice/chicken mix for another small meal, etc.
Has your dog eaten something it should not have?? How long has the diarrhea been going on? Is there any vomiting?