You have to do everything you can to lower nitrites. If you can, remove all the other fish in the tank (so there is less waste produced). Any uneaten food whould be siphoned up after 30 mins max - try offering him frozen bloodworm, but any not eaten will need to be removed quickly. You must also do frequent water changes - say 25% per day, even twice a day. The reading isn't ridiculously high, but discus are really sensitive to any water quality issue.
IMO you need to concentrate on the water, and get him back to health before considering your next move re: tank size and companions.