If this has just happened, you can try inducing vomiting with a fresh bottle of 3% hydrogen peroxide (this is the kind found at the drug store or grocery store for cleaning wounds, not the stronger type that may be used for dying hair, etc). For a 40lb dog you can give up to 40 ml (this is 8 teaspoons) orally, and if no vomiting occurs, you can repeat this one time.
Without knowing the approximate amount of snickers eaten, it is hard for me to tell you if chocolate toxicity is a concern here. Luckily snickers are usually just coated in milk chocolate, so it is less of an issue than eating say a solid hersheys bar, but there is still some risk. As I said, without any idea of the weight, it is hard for me to tell you if it is a potentially toxic amount. There is a toxicity calculator at http://www.petmd.com/dog/chocolate-toxicity if you do find out the approximate weight of the bag, just realize that they amount of chocolate is going to be less than the total weight of the bag because it is just the coating that is chocolate.
Just the ingestion of any new food can cause vomiting and diarrhea. Chocolate toxicity can also cause racing heart rate and irregular heart beat, restlessness/hyperactivity, twitching/tremors, and seizures. If any of these were to occur, you need to get him to an emergency clinic ASAP. An article on chocolate toxicity is here:http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=3544
The other concern here is if he ate the bag and the candy bars with the wrappers intact, is the possibility of an obstruction. For this reason, if your dog is having continued vomiting, abdominal pain, or inappetance, then he needs to be seen by your vet.