[Willow Wisp]

I'm in the UK so don't know, what type of chocolate are Hershey bars?
Milk chocolate or plain?
Plain chocolate is the most dangerous, although obviously milk chocolate isn't good.
As others have suggested, empty the cage to ensure none is stashed.
Vets do have an antidote for chocolate poisoning, I believe it's charcoal, and an iv drip would be needed too but in such a small animal speed is of essence, at the very first sign of any unusual behaviour don't waste a second before getting him to a vet.
Speak to your siblings, hopefully they only put in the amount you've found and none has been eaten, good luck.