Is he the little one in your avatar? I looked for another thread you might have made but it looks like you've only made one post at all on this forum, which was the post you made to start this thread.
If he's the duck in your avatar, it looks like he's a domestic duck, so he definitely shouldn't have been at the pond and was probably dumped there by someone who thought it was fine to do that and he would survive. Domestic ducks can't survive in the wild, so you saved his life.
It looks like he has a beak injury, so I would clean it off and apply some neosporin to it.
The description you're giving of him not being able to walk well makes me suspect a niacin deficiency, especially since he appears to be a pekin, and they tend to need a lot of niacin, and there's no telling what the person who dumped him was feeding him.
Go to a feed store and pick up something called Vitamin B complex, and give him 1 mL a day directly, either via a syringe or over a treat like peas or mealworms. This should help.
Also while you're there pick up some food for him, an all flock like Purina Flock Raiser if you can find it, or Naturewise also makes a feed that will work. It's called Meat Bird and comes in an orange bag, and they make it with ducklings in mind. This is what I use for my ducklings.
If you can't find either of those, go with an unmedicated chick feed but make sure you're religious about supplementing the niacin with the B complex. If you can't find unmedicated, then medicated with amprolium is fine, it won't hurt him.
