My first thought was what Filarotten said. Just print out a copy of the article on grapes and give it to him, that way it's not you telling him something. Then, if he's testy about it or still tries to feed her grapes or raisins you can tell him you guess he's purposely trying to kill your dog . . .
Or maybe you'd better ignore that last part, lol! That's definitely something I would do - and the kind of thing I did when I was your age (and still do on occasion), but I was always willing to take the slap in the face for it