The toxic dose of grapes to the weight of the dog is
32 grams per kilogram body weight. 32 grams is
approximately one ounce. A kilogram is
2.2 pounds. I don't know the size of your dog
or the amount of grapes your dog
has consumed, but a toxic dose for a 22 pound dog
(10Kg) would be 10 ounces. Grape toxicity can cause acute renal failure.
If he ate the grapes just a few hours ago you may be able to induce vomiting
by forcing him to drink a couple of ounces of hydrogen peroxide.