Definitely x2 on spending more time near the tank. Grab something to do and sit in a chair near him for at least half an hour a day, longer if you've got the time. Don't wear anything red, bright or shiny, or that looks like a face or a fish. Once he's gotten used to you for a few days, you can try other things:
Always wait to feed him until he approaches you, so he learns that bravery will be rewarded. Open the lid and sit there with food at the ready. When he peeks out of his cave, drop in a pellet/worm whatever.
Gently tap the top of the tank before dropping in a piece of food (Never overfeed! Just add this to your daily ritual) so he learns to come out on command.