Hi BethEmma, congratulations on your new family member! He's a good looking dog. It is wonderful that you have adopted a dog who needs medical attention, and that you care so much about him.
I've never had a dog that tested heartworm positive. Thankfully, heartworms are treatable, but it must be tiring for a dog to go through. It takes about a month for the heartworms to die, I believe.
Your new family member needs his R&R. Now that he is in his own, new, loving home he'll start getting more rest., and a good night's sleep.
Many years ago I worked at a vet in the reception area. Someone left a dog overnight and I brought her home that night. The vet said expect her to be up crying all night, because she was in a new place. But she slept like a log through the night. Before I brought her home the vet got rid of her fleas, worms, she had ear mites, a UTI, a bladder infection, and mange. No wonder she slept like a baby. She finally felt comfortable enough to sleep.
Try contacting the shelter and ask them how long it will take for him to get his energy back. They are used to dealing with heartworm positive dogs. Maybe it depends on the dog's age, general health, and severity of the heartworms?
You could also talk to your vet and ask him or her how long it will take to get his energy level back up. Maybe there are vitamins or something with added nutrition that can help him to get his energy back sooner?
He's been through a lot, but finally is with his forever family.
I wish you all the best, and a speedy recover for him.
#PetHealthandNutrition