Baby Duck less then a week old troubles

So my duck is less then a week old. Earlier in the week I asked for help because of his legs. Poor thing kept falling on his face and it did not seem right at all. To say the least everything is underdeveloped with this little guy. I'll share 2 photos. One is of him with the another duck that hatched a day after he did. We named them Midget and Goliath I'm sure you can tell who is who. I know Midget is eating, and drinking, he acts like a happy healthy duck. Maybe not as much energy as the other one but I thought it might be because he is so much smaller. But he does run, swim, and play. He has started to walk a lot better then before. But this spot on his belly worries me. I thought it was just from hatching when I first saw it. But I think it's getting worse. What's wrong with Midget? :-(