That's definitely a tough one. Before I knew anything about goldfish I had a two inch comet. Then I decided to get a black moor and a lionhead that were bigger than him. My comet was so scared that he wouldn't come out of hiding for four hours. Now he's almost twice as big as the other two and big piggy!

I notice that he eats fast and quick so I always give a little extra for him. Also, I notice that my black moor will follow the other fish to get food. So, despite my black moor being slower he has developed a system to get food. Food goes in the tank, the other two goldies go nuts, and he follows them!
Yeah, I would try feeding him at the opposite side of the tank. Of course I tried that with my fish and that durn comet Frankie just zooms accross the tank and always sees the food!