Try to observe him for awhile without the fish noticing that you are watching. If you have more than one, in all liklihood they are picking at each other, even if he is the dominate one.
In the meantime, drop the flake food, and add a lot more nutritious variety to your fish's diet to include a variety of frozen and high quality dry foods. You should feed at least twice a day. To the frozen foods, add a couple of drops of zoecon to the food before feeding.
Let us know if this resolves the problem.