Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Boomer ingested some grapes, but pleased to hear that he didn't eat many and he is a big fellow.
Dogs must eat a certain amount per pound of body weight to get sick. Grapes can cause nausea, vomiting, diarrhea, then increased water consumption, lethargy, and kidney failure in dogs, but the toxicity is dose dependent meaning a big dog would have to eat more than a small dog.
Some dogs are more sensitive than others but a toxic dose is usually 3 grapes per kilogram of body weight (3 grapes per 2.2 pounds).
In very sensitive dogs they report that dangerous doses begin at about 0.18 ounces of grapes per pound of dog weight. There are about 5 medium grapes per ounce, so that means about 1 grape per pound could be toxic.
If he weighs 110 pounds he should have to have eaten more than 110 grapes to see signs of toxicity.
I understand there is a lot of conflicting information out there. To be safe we recommend no grapes. But they do need to eat a certain minimal amount per pound of body weight.
I always recommend that owners watch their pups for vomiting and/or diarrhea several hours later just in case he ate more then you thought he did. Signs of toxicity usually occur with 12 to 24 hours, but in some cases 48 hours. Further signs of toxicity include lethargy, lack of appetite, abdominal pain, increased water consumption and finally lack of urine production.
I tell owners if they see vomiting and diarrhea with any amount of grapes fed we need to get them on intravenous fluids to protect the kidneys.
Best of luck with Boomer, please let me know if you have any further questions.