Hi Sir or Madam,
Your puppy needs to be seen by a vet immediately if his gums and tongue are gray and he is lethargic. A pup this age should not be at a park or be around other dogs until all its vaccinations are done which isn't until around 12 weeks of age as this potentially exposes him to parvovirus and other deadly diseases. . If it was a very timy amount of a chocolate cookie, it is doubtful that it has enough chocolate to be toxic. However, it certainly isn't doing him any good. What you are seeing is likely from the rat poison.
Since he was at a park, he may have been exposed to diseases such as parvovirus, so you will need to keep an eye on him for that in the future as well. However, for right now, the pup needs to see a vet immediately. Please do not delay. I hope this information is helpful to you.