Seems like you have the bases covered- sounds more like he's still not comfortable. When you come close to the tank how does he act. Does he move his fins frantically or in a smooth pace. I would just work on getting him used to you. Hopefully, the tank is set up on the basement floor or solid ground, so the fish feels no vibration as people walk, talk, etc. Take the hiding spots out and then try to mess with the tank as little as possible. Do regular water changes without spooking him too much, he'll realize the freshwater and start appreciating it. Sit and stair at him everyday for about and hour. Put a mirror up during the day so he can shadow box. Be consistent with the times you turn the tank light on and off. Try turning on a lamp or other light for about 5-10 minutes before turning on the tank light. Put a few clean (treated) live guppies, or rosy reds, like once a week or brine shrimp. Maybe even small crayfish that have been bagged in saltwater from the store to your house, to be safe. Nothin like live foods to bring the beast out! Eventually switch to krill, that'll get him. Last but not least, he may just be a shy fish. He may do better in a community tank than by himself, as long as the other fish are a little bigger and wont kill him- oscars, peacock bass, managuensis, etc. look up the term "dither fish."