The water you're taking from your cycled tank... are you sure it has 0 ammonia? When you do water changes, are you siphoning out the gravel or only removing water? How large are the water changes you're doing?
I'm very sorry this is happening. In the future, bettas usually respond well to frozen foods like bloodworms, mysis shrimp, and brine shrimp. Thaw them, then offer them with a large eye dropper.
I'm not sure he had internal parasites. Internal parasites would mean he was eating, but was loosing weight and his poor would be white and stringy. I think he was underfed at the store, exposed to constant ammonia at the store, exposed to ammonia at home, became lethargic and denied dry food.
I have had something similar happen, I rescued an underweight veil tail and he appeared to get better. He was de-wormed, gained weight, got energy, then... In a few days he went from lethargic to dead.