http://bigdogsbigheartworm.blogspot.com/2011_06_01_archive.htmlWelcome! There's a crap-ton (official measurement) of information on heartworm on that blog, starring a girl right here on this forum, Mila, who is now living the life!
What probably happened, with a dog who is at 10 months and HW+, is that they may not have tested - depending on when they got him into rescue, because typically puppies under 6 months (check that age - it was a while ago that I read that stuff) aren't alive long enough to have adult heartworms because of the length of the life cycle, and the monthly medicine kill any baby worms that are there. But, if they were giving something like Heartgard, and he swallowed it whole...it's possible to have the adults, I am guessing.
Biggest thing, whether you use the Immiticide treatment
http://bigdogsbigheartworm.blogspot.com/2011/08/urgent-post-immiticide-unavailability.html , or the Heartgard/Ivermectin, is full on crate rest/exercise restriction.
Ask your vet about doing the doxy before treatment starts. All of it's in the blog, or should be, and if you want to start a post in health, we can all contribute questions to ask your vet.