I loved my bi-color, named Bob, so I understand where you're coming from. Bicolors will eat just about anything once they get established at feeding but try tempting him out with some small meaty morsels. Once he gets used to coming out to feed, he shouldn't be so shy.
I'll bet though, that he's eating algae off the rocks and glass. Mine used to do that all the time. I miss ol' Bob; I lost him when I had a dinoflagellate bloom in the tank.