Has he got plenty of shelter to make him feel more confident? And do you let the day come gradually, first turning on the room lights and then the tank lights 10 minutes later? The other thing that makes corys more confident is having other corys around, as they are schooling fish and should not be kept on their own- but of course, this may not be practicable if there is no tank space. Have you had him long? My corys were quite easily spooked when I first got them, but now when I've had them 2 years, they take things in their stride.