OMG that is scary - I hope he's ok !!! Have you tried a syringe to feed him water ? I don't know if you should try Nutrucal for him if he's not eating but it sounds like a vet visit is the way to go today ...poor baby - you really ARE lucky - grapes can be fatal....thank goodness he only had one !
When I first got my Chanel 3 years ago ...I looked at many sites about toxic foods - I still had the search from looking at it recently ...and you can type TOXIC FOOD FOR Dogs/CANINES -
There are many everyday things that dogs just cannot eat - THIS Page has a great list to start you off -
http://www.risingwoods.org/OURFAMILY...odsfordogs.htm
GOOD Luck !!! I hope he gets to feeling better and has no after effects from this - Please let us know what the vet says