I don't know how or why you thought he had parasites.. from a white belly?
You do realize that sometimes when a betta's belly is extended it can turn a lighter color, right?
Must likely you are over feeding him, and/or feeding him lower quality food that is causing the bloated look.. some extension is fine after a meal.
The reason his belly doesn't look bloated anymore is pretty much because you forced a laxative on him and he expelled what was in him.
Not all bloated bellies means there is anything wrong with the fish.. I have an old male who with age, his back has curved downwards a lot causing his belly to look really big. If I threw his photo up here people will start telling me he has six different diseases.
Not your fault that you were told it could be parasites without any actual signs of parasites.. but would be careful in the future. Also, never do treatments in the home tank unless it is something such as ich, velvet or a fungal attack.. luckily ES salt is gentle enough for the bettas to live in it indefinitely, but it doesn't mean they should, and with any salt treatment you should be doing daily 100% water changes, why a small hospital tank is a must when owning fish.
Good luck with everything, glad he is doing better.