Just get as much peroxide in him as you can.
Some dogs will puke after a tablespoon full, some will take a cup and occasionally some won't puke at all. Pour some down, wait 5 or so minutes and if no puke, pour some more down. In this case, if no puking within 30 minutes, I would take the dog to the Vet because of the grapes.
That is just my thoughts. I'm not a Vet nor did I stay at a Holiday Inn last night.