I agree with ginger, I had a dog as well who used to eat grapes years before learning they were bad. but if you are concerned a vet trip couldnt hurt. but be sure to watch how much he is eating and drinking, you want to make sure he doesn't get dehydrated. good luck!
I am sure he'll be OK. I had a Jack Russell once (pretty small dog) and in those bad old days I didn't KNOW grapes were bad for dogs and can affect their kidneys. I regularly gave him grapes, thought they were good for him! Can you believe that?
Anyway he was never sick from them, had good health, and didn't develop kidney issues. He lived to be 14.
I think so long as you remember not to give your dog any more, and so long as he doesn't show any sudden signs of being unwell, he'll be OK.
I don't think you have to worry until you see some ill effects. You say he isn't sick, healthy stools, not throwing up. Eight grapes aren't that bad, some dogs will eat some very weird things. How old is he and what is his weight?