sick duckling

is the duckie crapping alot? or does he just sit there? Im thinking coocidia or some other kind of infection. No matter what, you should separate the sick one from the others so in case its communicable (coccidia can kill a whole flock in a few days)

The reason I said coccidiosis is because you described one of the symptoms we call "tucking" , they keep their heads and legs close to their body . Often theyll have blood in the stool . If that happens, the bird is probably too late to help.
Just keep it separate from the others OK? Thats always good practice when youve got any sick animal in a herd or flock. It prevents the others freom preying on the sick one and it also keeps them more free of infection.

Nature is cruel and your job, as the keeper, is to not help nature in her job.

I hope your duck is still with us. You should check your posts to see whether anyone has responded with advice. If itd be me, Id be trying everything from the poultry manuals.