It would help to know how many he had. But I would call the vet to be safe.
FYI, most chocolate (hershey's kisses included) have very little of the actual ingredient that is toxic to dogs. Before I knew better, my 7 lb peke mix ate M&M's all the time and she lived to be 15. Not a lot of them, but a few everytime I bought some til someone told me they were a no-no. I've also known a Aus Sheperd that ate about a 1/2 pound of fudge that was left on the counter. The vet advised to just watch her for signs of lethargy and/or vomiting. Nothing happened, except I had to make a new batch of fudge for the office. :rolleyes:
Do a search here on "chocolate" and I am sure you will find more.