i did some research regarding this: i understand now. "Single-sex heartworm infections are more common in cats; microfilaria are seen in less than 20%, and they are only present for about a month. The microfilaria are then swallowed by biting mosquitoes, and develop into infective larvae in the mosquito in ten to forty-eight days, depending on the climate. The larvae move into the mouth parts of the mosquito, where they can infect a new host--a cat or dog--when the mosquito bites the animal." From what i understand, you're correct it's usually a new infestation. However, it does state that regular heartworm preventative can be used to kill the majority of them so now i see why it's not optional to give him heartworm preventative.
thanks for bringing this up. i learned something new!
also, many dogs live long happy lives after being infested with heartworms so i wouldn't really put it off as him being ill. just be glad that someone so nice like yourself took him and now has taken the measures to prevent him from worsening his condition!