If he is rubbing against tank decorations, or substrate scraping himself, plus the white stringy poo then it does sound like an internal parasite. But if he is eating, and swimming normally, then that is a good sign.
I would do a large water change, raise your temperature to at least 85 degrees, add 1 tablespoon of salt per 5 gallons, and continue with the internal parasite medication.
You should invest in an API Master Test Kit, also.