The amount of toxic
substance from chocolate that is present in a chocolate chip cookies (or even a few of them) is usually not enough to cause a true chocolate poisoning problem for a dog. Most dogs who suffer from chocolate toxicity, do so from eating baker's chocolate. However, since your dog is so small, these chocolate chip cookies could be bothering him. I still doubt that he could have ingested enough chocolate this way, to be considered truly toxic.
Signs of chocolate toxicity include vomting and diarrhea with in the first 2-4 hours after ingestion, fever, restlessness, hyperactivity, and increased urination. Advanced signs include stiffness, excitement, and seizures
Even without toxicity, your dog will likely experience some diarrhea. If this should occur, I recommend feeding
a bland diet of boiled white rice and boiled skinless white-meat chicken
(mixed 3 parts rice to 1 part chicken) until the stool firms up. You can then slowly start to mix back in the dog's normal diet.
If any of the more advanced signs should appear, then you need to contact your veterinarian immediately.
I hope that this information is of help to you. Please let me know if I can be of further assistance.