Unfortunately it's against forum rules to offer medical advice for a dog that hasn't been to a vet. Heartworm treatment really needs vet intervention, because the treatment can kill the dog if it's done incorrectly. I would honestly look into whether you could report this dog as a neglect case to your local animal control, with the information that he's heartworm positive and has a paw injury, but is receiving no treatment for either. If you don't want to go that route, I'd start contacting vets and shelters/rescues in the area to see if any of them know of charities or reduced cost vet services that might be a more viable option.