The latest verdict is that it isn't the grapes, it's the chemicals (man-made and natural) that's on the grapes that are the problem (yes, I can provide supporting info if you want).
If you washed them before you put them where he could get them, then you quite probably don't have a problem ... but as Jenn says, keep an eye on him.
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“Properly trained, a man can be dog’s best friend.”
Corey Ford .