[sarahr]

I would defineatly put him on heartworm, if he get's that is some really nasty stuff. it is better safe than sorry. I used to work at a vet's office, and i saw plenty of dogs get heartworm, it is not good at all, not to mention how expensive it can be to treat. (All it takes is just one misquito bite.) Heartworm med. is actually pretty cheap around here, i don't know about where you are.
It's the frontline that is usually more expensive, i give my charlie that too, because my mom didn't give it to her dog and he got bite by a deer tick and got lymes disease, he could barely walk. He got better, but he still limps sometimes. I would just put them on the meds, because i would go crazy if charlie got sick.