Pepper - I'm so sorry LD has to go through this. I'm sure he will recover because the treatment is very effective. Many of the pups that come to Yankee Golden Retriver Rescue have heartworm and they are treated very successfully before they are ready for adoption.
The best thing against heartworm is prevention. Ask your vet about Heartgard or any other medication he can recommend. The disease is transmitted by mosquitos and since the weather has been so warm until now, it's possible that there are still mosquitos around where you live.
We'll keep LD in our thoughts, and we send him many good wishes for a quick recovery.