Yes, I’d get the dog to the vet immediately.
After reading Scared Poopless by Jan Rasmusen we decided not to give heartworm meds because they are basically poisons meant to kill the heartworm/larvae.
http://www.dogs4dogs.com/blog/2009/05/13/heartworm-medication-safety/
For prevention we always apply natural flea/tick spray like Mercolas, and test twice a year for heartworm to get it early when it’s not dangerous and treatment is easy. From what I’ve read, it’s not as easy to get heartworm as vets make it out to be.
How is he doing?
