I am sorry to hear you are worried about Wriggley. I am a veterinarian, and will do my best to help.
He will probably be fine. The pills give a dose of around 6 micrograms (1/000th of a milligram) per kilogram (2.2 pounds; we calculate drug overdoses in micrograms per kilogram) of body weight. If he got 5 pills, he got 30 micrograms per kilogram.
In dogs that do not have the MDR gene, the minimum toxic
dose starts around 400 to 500 mcg per kg; he is well under that. If he were a collie
, or Aussie with the MDR gene, we would start to worry around 50 mcg per kg, but he is well under that, as well.
I would not expect any problems, and would not rush him in to your vet. You will have to get more heartworm preventative and give him another dose next month, however.
Let me know what else I can help with or answer.