If he's pecking at the rocks looking for food but not eating what you give him, try this: Take a small piece of algae sheet and tie it to a small rock with a rubber band then set that on your rocks and see if he eats it. If that works, keep doing it for a few days so he gets used to the sent and taste. Then try a piece of algae sheet on a veggie clip and the rock at the same time. If that works for a few days, try just the veggie clip.
Good luck.