Hey Daffy! I am new to being a duck mom as well. because I am so new at all this I am just guessing on what the problem is and it's very common! it is called splayed legs, or spraddle legs. it can happen for a few reasons. 1.) shortly after they are born they can slip on a wet surface do a split and their muscles have not developed properly. 2.) after they hatched they were already bowlegged and the problem has continued to get worse because it could not walk properly. 3.) if the ducky is being a little piggy... and over eating and gaining too much weight too fast! his small and weak developing legs could not support his body weight & his legs are spreading.
the good news is that this is treatable! more good news! because he is still very young his muscles (are not already fully developed) you can correct the problems so as he continues to develop his muscle, He can properly with extra support.
I believe you can search for splayed legs on backyard chickens and will be able to find many people who have gone through the same thing! they will probably have figured out the best way to go ahead in fix the problem. Ive seen images of people using bandaids all the way to making cute little handcuffs out of pipe cleaner. you will basically make a handcuffs out of the pipe cleaner and attach below the ankle and above foot the foot itself. making sure it is not too tight, shorten the distance between the handcuffs every few days. it can take anywhere between 7 to 30 days to correct the problem. on another note real quick I think it is very smart of you to have brought the little duckling inside!! He could get knocked over and problem could get worse but with him already being a week old and being around his siblings and mom, he is going to be miserable and lonely. I do not know how many ducklings you have but I would definitely bring a friend in for him. it would help his morale, his anxiety, and it would make it easier to reintegrate him after a few days away from the gang! I am sure there is a forum on here about Splayed legs. you could also check Google out and they had a lot of information and different photos of different people making handcuffs for chicks and ducklings. I hope this information helps and the little guy gets better soon!!