BTW, the way to induce vomiting in dogs is to administer hydrogen peroxide, the same bubbly stuff that you buy at the drugstore to disinfect wounds. There are a lot of sites on the web that will tell you how to do it, if you google "inducing vomiting in dogs," but it is probably better to ask your vet on a routine visit when you are there for something that is not an emergency. I had to learn how when my dog ate rat poison. So get that squared away before the need arises, and then you will be prepared, in case someday he eats anything else that he shouldn't.
I think he will be fine also. I wouldn't do anything about it. From what I understand, the dogs that have been poisoned from grapes were dogs that ate really massive amounts. As ginger says, nobody even knew for many years that grapes could be poisonous to dogs. I don't think one grape is a big deal, even for a small dog. I think you will just make it worse if you try to feed him hay or rabbit food, but it was good thinking.
I'll just add that my dog, Toby was a small dog, a Jack Russell. (By the way, he lived -kidney disease-free for many years after the grapes)
Years ago, well before I knew grapes and raisins were not good for dogs, my other dog, Toby used to love fruit. I gave him grapes (not many...about 4 or 5) quite often. Now I would never do that now, as I know better. But what I'm saying is he was fine, and showed no ill effects.