if you can't afford more items inside the tank, just sit something up outside of the tank that could be a point of distraction for him. I think it was Tiara here who suggested that. a pringles can, a potted plant, sometimes I prop a stuffed toy (borrowed from the dog) near the tank. something colorful that might interest him. make sure, however, that if he flares at it repeatedly, to take it away as it's irritating him rather than entertaining him.
and good for you for getting him into the 5 gal.