Mosquito bites do not necessarily mean heartworm risk, though. You could take steps to minimize exposure/attractiveness to mosquitoes - add garlic to the food during the summer months, use a garlic-based spray for the yard and for him before he goes out, especially in the peak mosquito times of dawn and dusk; there are also herbal combinations that repel mosquitoes. Here's an article about heartworms and some suggestions for mosquito repellents:
http://216.239.51.104/search?q=cache...org/gpage.html
(This is a cached page from google, so I'm not sure how stable the link is.)
Program is for fleas, though, so I wouldn't bother giving that.

If you must give heartworm meds, I would avoid as much as possible the "combination" ones like Sentinel or Revolution that treat for every possible parasite a dog can get - these rank highest among wormers for adverse reactions, probably because there are so many toxins given at one time. Personally I'd even prefer to avoid the heartworm/intestinal worm combos, but that can be difficult to do. Interceptor has received FDA approval for effectiveness at 1/5 the current listed dosage, so you could give the smallest size pills; studies also showed that heartworm meds (I believe both Interceptor and Heartgard) are effective for up to 60 days post-exposure, and equally so at 45 days as at 30, so an extended interval will help to minimize the toxin exposure.