How old is the duckling now?
If he has fully recovered from the effort of hatching and he is still all floppy (i.e. everyone else is fluffy and steady on their feet except him) then it sounds like it might be necessary to hand raise this little guy for a bit. Do you have a heat lamp or reading lamp and a box that you could use to rig up as a brooder? You can provide a small soft toy for company.
With any kind of leg or neck problems it can be extremely beneficial to give vitamins in the water - something like Sav-a-chick. Any kind of soluble vitamin meant for birds. It is even fine to use Berocca though! I'd make up some vitamin/Berocca water and give it to the duckling in an eye dropper. That, as well as keeping the little guy warm and safe, would be my priority.
With the legs, sounds like he doesn't even have the strength to get his feet under him. I'd try the vitamins for 24 hours and then try again with the leg taping. Make sure he is not on a slippery surface (if you put him in a brooder then an old bath towel is a good surface).
With feeding, see how he goes feeding himself without the competition of his siblings.