To keep your dog completely safe, do NOT allow him to eat any chocolates. Chocolate cintains the active ingredient theombromine, which can be toxic to dogs if enough is ingested.
I never feed my dog chocolate, even if she really wants it...
(a stranger gave her a whole chocolate once, becouse she was "sweet"[she didn't ask me, just gave it to Pami], and she had diarrhoea for 3 days... Some people...)
But one of my friend does, she eats a bar of chocoéate together with her dog, and theres nothing wrong with the dog, she is turning 9 this year, and she's compltely heathy. So I don't know, maybe it's different with every dog.