Has your dog vomited yet? Call your vet and ask if you can give your dog some charcoal,you can get it at the pharmacy. It's not clear why grapes and raisins are not good for dogs, but certain dogs can be highly sensitive to grapes, and others, not. The most acute symptom noticed in a dog who has eaten grapes is kidney failure, and I can't tell you how many grapes could do that. I know that a bag of raisins can certainly produce a toxicity that is lethal to dogs. How much does your dog weigh, and what breed is he?