[anonymously]

I would be more worried about the dog surviving the heartworm treatment, that is a priority.
Otherwise I would go by your vet’s suggestion, maybe a high quality puppy food (canned and dry). I might add a tablespoon of chopped cooked chicken or chopped cooked chicken liver, add a little water to meals. I might feed 3-4 small meals per day instead of 2 regular ones. I would not add any supplements unless your vet advises so, just more junk for the liver to deal with. I would not give raw food to an old sick dog.
You don’t want rapid weight gain, too stressful, just a gradual increase.
Also, see what your vet advises regarding the flea allergy, frequent baths? Be careful about vaccinations and pesticides with a senior dog like this….
Walks/activity as tolerated, keep him moving.