Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry that you have been waiting for a response, but your requested expert isn't online which delayed your question coming up on the list for all to answer. I would like to help if you are still interested in an opinion.
Although raisins and grapes are toxic for dogs they must eat a certain amount per pound of body weight to get sick.
The toxic dose of raisins in dogs is estimated to be about 0.1 ounce per kilogram (2.2 pounds) of body weight. For reference the small boxes of raisins that we put in kid's lunches are about an ounce. That means one of those small boxes would be toxic for a 20 pound dog.
If your pup is 20 pounds one raisin will not be toxic for your fellow, he would need to eat several more.
The problem with raisin toxicity is that by the time we see symptoms the damage is done. There isn't any true antidote, only supportive measures. In cases where we aren't sure how many were eaten, especially in a small dog that just ate them, I do recommend inducing vomiting with 3 % hydrogen peroxide.
If you are certain he only ate one however then this isn't necessary.
Signs of toxicity start 6 to 12 hours after ingestion. The initial symptoms are vomiting and diarrhea. If you do notice those, even if you think he didn't eat that many raisins, I would have him checked immediately.
If he had eaten a sufficient quantity I would be concerned about kidney damage, but I would expect gastrointestinal symptoms first.
And I would not expect any symptoms after eating one raisin.
Please let me know if you have any further questions.