Take him out and put him in a quarantine tank. Raise the temp to 90 and add a couple tablespoons of salt. Do a daily WC replenishing the salt each day. When he starts eating, reduce the salt over the course of a few days and get 2 more discus for the QT. In a month, introduce all 3 into the main tank.
And get (and keep) your main tank nitrates under 5.
P.S. gravel is bad for discus. Nitrates creep up too quickly and it harbors too much gunk no matter how much you vacuum.