Is he by himself? They also like to be in groups. At one point, I only had two. One was thriving, the other hid all the time, to the point I was afraid he was going to starve to death. Now I have 4, and they're all really active, even the shy one. He's still the smallest of the group, but he's gained some weight and looks healthy.
Mine love the frozen shrimp and blood worms. As mentioned, since their eye sight is so bad, they find their food by smell, so they eat much better than when I was only feeding pellets.
My small one is a terrible hunter, can't find food when he's sitting in a pile of it. I'm pretty sure the only things he can eat are the frozen foods, because he can't even find the pellets when they're in front of his face.