First of all have you had a gill scrape done to check that the parasites are gone? or if you are shore they are id turn up the amount of O2 going in ie more air stone and do some 10% dayly water changes and not offer him anything for 5 days then on the 5th day try him with a fince smell pice of fish should work. And already said keep the lights off for the 5 days as well but id turn them on for 10 mins just to check dayly.