Adorable!!!! He is just so sweet.
And no, this is not a huge duckling problem. It is just a problem that comes up in any kind of fowl because, let's face it, nature's approach to propagating animals like this is to make them in quantity, not necessarily quality. By which I mean, in the wild, prey animals (like most fowl) make lots of babies and they make them fast because they simply don't expect them all (or even most) to make it to adulthood. As a result, you get cripples from time to time. I think the average, if your incubation is correct and other factors are in line (such as nutritious feed for your egg layers), is for cripples to represent less than 5% of your hatchlings (in ducks). I've never raised chickens, but it's my impression that they actually have higher rates of deformities and death among hatchlings--ducks are quite hardy by comparison.
It is true, however, that when you *do* have trouble with ducklings, it is often leg trouble. It's their weak point, just like the stomach is a weak point in horses and hips are a weak point in large dogs.
Laurelm--Make sure you are supplementing Scooter's water (or feed) with niacin. Ducks have a higher need for niacin that is not met in chick feed (there is *some* niacin in chick feed--but not enough). And niacin has a strong impact on the strength of legs. You can either get some brewer's yeast and mix it in the food (which is the easiest thing, IF you can find brewer's yeast--usually at health food stores and always at brewer supply stores if you can find one), or you can buy niacin gel caps at the pharmacy and mix it in the water. If you use the gel caps, you must get the dosage right, as it can be toxic in large doses. The maximum is 500mg per gallon of water. I use 250 to be safe, and it *seems* to be enough in my ducks. Don't get the "flush-free" sort as it has extra chemicals, and the "slow release" type tends not to dissolve well. Just plain old ordinary 500 or 250mg niacin gel caps is what you want. Just break the cap open over the water and stir to dissolve. If you have separate drinking and swimming water, you can mix the higher dose (500mg) in the drinking water and leave the swimming water alone (since they will drink some of it, it will dilute the dosage overall).
Good luck to Scooter. He's already doing better than Chance was.