It's hard to know why the others are eating now and this one isn't. I know levamisole will cause discus to stop eating. It sometimes takes a few days for them to start eating again even after you stop the treatment. And I'm not convinced they ever had worms in the first place.
Just doing frequent, large water changes ( like 50% daily) for several days can make sick fish start eating and act normal again. Especially, if they have Hexamita.
Separating this fish from the others is a good idea, but isolating one discus in a tank by itself is also stressing it. Discus don't like being alone. That may complicate getting it to eat anytime soon.
If it doesn't start eating in a few more days, you may have to consider treating with metronidazole to see if this is a Hexamita infection. It's much more likely that's what they all have had since the beginning.
To answer your question, I would leave the temp. around 84-85 for now. If you decide to treat with metro, you will need to increase the temp during the treatment.