Pet rat hind leg weak - is he OK?

His leg problem could depend on his age. A young rat most likely had an injury you didn't know about and hurt his leg. Most animals tend to do just fine if a leg is hurt, just continue to make sure he is getting all his food and water. An old rat may have arthritis or some other problem that causes weakness of the leg. My hamster dislocated her hip and survived perfectly fine until we had her humanely euthanized because she was suffering and the vet said it was either that or a really expensive surgery adn she was already old. The rat should be fine just keep an eye on it. And about the Chocolate, like anything else its alright in moderation, maybe a little piece every other week is alright. It is also very common for male rats to lose back leg function in old age.