How long have you had him. Do you have a lot of plants in your tank as it is very unusual to have 0 nitrates in a goldfish tank? Is your tank cycled? Are you aware of the aquarium nitrogen cycle? Could he have gotten caught in anything? I would continue daily water changes and add Aquarium salt at 1 tablespoon per every 5 gallons of water the first day. Be sure to dissolve the salt first in a little tank water before adding to keep it from burning the fish. Continue to do a 50% water change daily for 2 weeks and each time add 1/2 tablespoon more of of aquarium salt. If he hasn’t hurt himself internally, this should help. He’ll be fine with the salt for 2 weeks. Aquarium salt acts as an astringent for injuries and fights bacteria while also reducing stress. I breed goldfish so I know quite a bit about them. Keep us posted.